Protein nanofiber air filter materials and methods

ABSTRACT

Air filters formed from mats of protein-containing nanowires are provided. The nanowires are formed into a mat with pores that allow air to pass through while physically filtering particulate matter. The protein in the protein-containing nanowires also serves to chemically filter polluted air passed through the filter. Specifically, chemical functional groups from the many amino acids that comprise the protein of the protein-containing nanowire react with certain chemical pollutants (e.g., carbon monoxide and formaldehyde) in order to capture or otherwise neutralize the pollutant. Accordingly, the single nanofiber mat performs two filtering functions. Methods of filtering air using the provided air filters are also disclosed, as well as methods for making the air filters from protein-containing nanofibers.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of PCT/US2016/054526, filed Sep. 29,2016, which claims the benefit of U.S. Provisional Application No.62/234,087, filed Sep. 29, 2015, the disclosures of which are herebyincorporated by reference in their entirety.

BACKGROUND

Air pollution has become a major environmental concern due to the hugeamount of pollutants produced from vast human activities. It containsnumerous combinations of pollutants such as particle matter (PM) ofvarious sizes, chemical mixtures, biological hazards, and etc. Moreover,creation of unexpected chemical compounds due to the photochemicalreactions in the polluted air, makes it more and more puzzling to cleanthe air. These complicated mixtures have posed excessive threats topublic health. PM contains small solid particles and liquid dropletswith different sizes. Regarding the size, particulate pollutants can becategorized by PM_(2.5) and PM_(10-2.5), indicating particle sizes below2.5 and between 2.5 and 10 respectively. PM_(2.5) is mainly one of themajor pollutants in many developing countries. These particles arecommonly composed of organic (e.g. carbon derivatives species such ascarbon oxides) and inorganic (e.g. nitrates, sulfates, silicates, etc.)compounds which can seriously influence the air quality, public health,climate change, air visibility and so on. In addition, polluted airincludes numerous types of toxic gaseous molecules, such as sulfuroxides (SO_(x)), nitrogen oxides (NO_(x)), carbon oxides (CO and CO₂),formaldehyde (HCHO), methane (CH₄), and a mixture of other volatileorganic compounds (VOCs). These chemicals can undergo variousphotochemical reactions which may lead to the creation of unexpectedhazardous pollutants. Biological hazards including bacteria, viruses,mites, pollen and etc. can trigger many allergic reactions andinfectious illnesses such as influenza, measles and chicken pox. Becauseof the intensive effects of these pollutants on the environment andhuman health, providing an effective protection, particularly towardimproving the indoor air quality, is urgently needed.

Filtration membranes are commonly used to remove the pollutants from theair and improve the quality of the air. Some attempts have been made forenhancing the outdoor personal protection, and improving the indoor airquality. An ideal air filter should have a high removal efficiency ofpollutants yet maintaining low resistance to the air flow. Conventionalair filters are usually made of micron-size fibers of synthetic plasticssuch as polyethylene and polypropylene. These air filters areineffective for removing the toxic gaseous chemicals from the air due tothe lack of active functional groups in the structure of the rawmaterials. These materials are only effective for capturing particulatepollutants based on the four primary physical and size-based filtrationmechanisms, including sieving, interception, impaction, and diffusion.

In view of increasing global pollution, a need exist to providefiltration materials that filter both particles and chemical pollutantspecies, while not significantly reducing air flow.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

In order to address the need for improved air filtering, we have foundthat the protein-based nanofiber (“nanofabric”) materials can providemultifunctional air filtration capabilities with very high affinity tovarious pollutants. These protein-based nanofiber filters demonstrateextremely high removal efficiencies for both solid particles withdifferent sizes and various toxic gaseous chemicals while maintaining avery low resistance to air. These capabilities make it possible to usethin layers of the protein-based nanofiber materials to develop highefficiency air filtering materials for practical filtrationapplications.

In one aspect, an air filter is provided. In one embodiment, the airfilter includes a porous nanofiber mat configured to filter particleshaving a diameter of about 0.1 μm or greater when air is passed throughthe air filter, the porous nanofiber mat comprising a plurality ofprotein-containing nanofibers, comprising a protein configured to bindto, and thereby filter, at least one chemical species.

In another aspect, a method of filtering air is provided. In oneembodiment, the method includes passing air through an air filter asprovided herein. As noted previously, the disclosed air filters formedfrom porous nanofiber mats are intended to replace present air filtersin essentially any application. Accordingly, the method of filtering airbroadly include any application of the disclosed air filters forfiltering air, wherein the air includes particulate matter and chemicalpollutants, both of which are filtered (at least partially) by theprotein-containing nanofibers.

In another aspect, a method of making an air filter according to thedisclosed embodiments is provided. In one embodiment, the methodincludes electrospinning a solution comprising a solvent and a precursorto a protein-containing nanofiber. Electrospinning is applied in thepresent embodiments to solutions that form protein-containingnanofibers.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1. Schematics of gelatin solution preparation and nanofibersfabrication via electrospinning, followed by the schematics of a singlegelatin nanofiber with a functional surface due to various functionalgroups available in its structure. Porous structure and fiber diameterof nanofabrics contribute to particulate filtration while the richfunctional groups on the fiber surface provides toxic chemicalfiltration property.

FIGS. 2a-2d . SEM images of gelatin nanofabrics prepared in differentratios of AA-to-water; effect of solvent ratio on morphology and fiberdiameters of gelatin nanofabrics compared with that of commercial HEPAfilter; percentages in the table are relative standard deviation.

FIGS. 3a-3h . (FIGS. 3a, 3d ) SEM images of gelatin filter withdifferent magnifications along with photograph of gelatin air filterbefore filtration, (FIGS. 3b-3f ) SEM images of gelatin filter withdifferent magnifications along with photograph of gelatin air filterafter filtration. Particulate removal efficiency of gelatin filternanofabrics; (FIG. 3g ) Particulate removal efficiency of gelatin airfilters with different areal density for various PM particle sizes.(FIG. 3h ) PM_(2.5) and PM_(10-2.5) removal efficiency comparisonbetween gelatin air filters with different areal density and commercialHEPA filter.

FIGS. 4a-4d . Chemical removal efficiency of gelatin filter nanofabrics;(FIG. 4a ) Formaldehyde (HCHO) removal efficiency comparison betweengelatin air filters with different areal density and that of commercialHEPA filter, (FIG. 4b ) Carbon monoxide (CO) removal efficiencycomparison between gelatin air filters with different areal density andthat of commercial HEPA filter. Pressure drop and overall air filterperformance evaluation of gelatin filter material; (FIG. 4c ) Dependenceof pressure drop (air flow resistance) on the areal density for thegelatin nanofabrics, and (FIG. 4d ) Quality factor comparison withcommercial air filters and transparent PAN air filter, and gelatin airfilter.

FIGS. 5a and 5b . (FIG. 5a ) Pollutant weight percentage gain (W_(p))changes of the gelatin air filter and commercial HEPA filter over thefiltration time, and (FIG. 5b ) pollutant weight absorption rate (W_(p)_(_) _(rate)) of the gelatin air filter and commercial HEPA filter overthe filtration time.

FIGS. 6a-6e . (FIG. 6a ) Simplified representation offunctional-capturing filtration mechanism of gelatin nanofabrics viainteractions between gelatin molecules and pollutants. (FIG. 6b ) SEMimage of gelatin nanofabrics after filtration (FIG. 6c ) FTIRcharacterization of cigarette smoke PM particles demonstrating existingfunctional groups. (FIG. 6d ) FTIR characterization of gelatin filterbefore and after filtration showing the active functional groups andPM-filter interactions. (FIG. 6e ) Functional group peak intensitycomparison between gelatin fibers before and after filtration indicatingstrengthening of corresponding bonds due to PM-filter interactions.

FIGS. 7a-7f . SEM images of gelatin filter nanofabrics with differentmagnifications (FIGS. 7a-7c ) before filtration test and (FIGS. 7d-7f )after filtration test showing the pollutants were grabbed around thefibers and deformed.

FIGS. 8a and 8b . Testing the functional group movements in the gelatinnanofabrics structure, FIG. 8a dielectric measurement, FIG. 8bpermittivity measurement.

FIGS. 9a-9c . SEM images of gelatin air filter nanofabrics after beingtested with cigarette smoke showing the deformation and migration ofsoft PM during saturation procedure, FIG. 9a first stage, FIG. 9bsemi-saturated stage, and FIG. 9c saturated stage.

FIGS. 10a-10e . Denaturation of soy protein. (FIG. 10a ) Schematicdemonstration of pristine SPI powder, (FIG. 10b ) SEM image of pristineSPI powders, (FIG. 10c ) schematic demonstration of denatured SPIparticles, (FIG. 10d ) and (FIG. 10e ), SEM and TEM images of thedenatured SPI particles, respectively.

FIGS. 11a-11h . SEM images of SPI/PVA air filter nanofabrics before(FIGS. 11a, 11c, 11e ) and after (FIGS. 11b, 11d, 11f ) filtration withdifferent SPI/PVA ratios: (FIGS. 11a, 11b ) 0:1, (FIGS. 11c, 11d ) 1:1,(FIGS. 11e, 11f ) 1.5:1, Inserted digital photos are the air filter withSPI/PVA ratio of 1:1 before and after the air filtration testing. Scalebar: 1 μm. g) fiber diameter distribution and FIG. 11h pore sizedistribution of SPI/PVA air filter (SPI:PVA 1:1).

FIG. 12 Particulate removal efficiency of PM_(2.5) and PM_(10-2.5) forneat PVA and SPI/PVA air filter nanofabrics with different SPI content.

FIGS. 13a and 13b . Particulate removal efficiency of SPI/PVA airfilters with different areal density. (FIG. 13a ) Particulate removalefficiency for various PM particle sizes. (FIG. 13b ) Effect of arealdensity on the PM_(2.5) and PM_(10-2.5) removal efficiency.

FIGS. 14a and 14b . Formaldehyde and Carbon monoxide removal efficiencyfor neat PVA and SPI/PVA nanofabrics: (FIG. 14a ) effects of SPIconcentration, and (FIG. 14b ) effects of area density for SPI/PVAsample with ratio of 1:1.

FIGS. 15a and 15b . Pressure drop and quality factor of SPI/PVAnanofabric filters. (FIG. 15a ) Pressure drop (air flow resistance) forthe optimized SPI/PVA air filters with different areal densities, and(FIG. 15b ) Quality factor comparison among regular commercial airfilter, commercial HEPA filter, transparent PAN air filter, and SPI/PVAwith ratio of 1:1 and 4.50 g m⁻² for areal density.

FIGS. 16a-16c . Study of the time-dependent behavior of the filtrationperformance for the SPI/PVA nanofabrics: FIG. 16a PM_(2.5) filtrationefficiency, FIG. 16b toxic chemical filtration efficiency, and FIG. 16crelative weight-gain of pollutions.

FIGS. 17a-17e . Filtration mechanism study of the PVA/SPI nanofabric.(FIG. 17a ) Simplified representation of interaction-based filtrationmechanism for soy protein-based nanofabrics; (FIG. 17b ) SEM image ofSPI/PVA nanofabrics covered by pollutants; (FIG. 17c ) FTIRcharacterization of cigarette smoke PM particles demonstrating thefunctional groups; (FIG. 17d ) FTIR characterization of neat PVA andSPI/PVA nanofabrics before and after filtration. (FIG. 17e ) Change ofthe peak intensity of functional groups/interactions responsible for thepollutant-nanofiber interactions.

FIG. 18. Particulate removal efficiency of PM_(2.5) for SPI/PVA airfilter nanofabrics with different SPI content fabricated viasolution-based/denatured-based and powder-based methods. From thisfigure, it can be found that solution-based method shows much higherremoval efficiency than that of power-based method.

FIGS. 19a and 19b . Toxic Chemical removal efficiency of FIG. 19aformaldehyde and FIG. 19 carbon monoxide for SPI/PVA air filternanofabrics with different SPI content fabricated viasolution-based/denatured-based and powder-based methods. Similar toparticulate removal efficiency, solution-based method shows much higherremoval efficiency for toxic gases than that of power-based method.

FIGS. 20a-20f . Pore size and distribution of SPI/PVA nanofabrics withSPI to PVA ratio of (FIG. 20a ) 0.6:1, (FIG. 20b ) 0.8:1, (FIG. 20c )1:1, along with that of (FIG. 20d ) neat PVA nanofabrics, and (FIG. 20e) commercial HEPA filter, (FIG. 20f ) Illustration of the way todetermine the pore size (pore size=(d1+d2+d3)/3).

FIGS. 21a-21e . Distribution of the fiber diameter for SPI/PVAnanofabrics with SPI to PVA ratio of (FIG. 21a ) 0.6:1, (FIG. 21b )0.8:1, (FIG. 21c ) 1:1, along with that of (FIG. 21d ) neat PVAnanofabrics, and (FIG. 21e ) commercial HEPA filter.

FIGS. 22a-22c . High magnification of the SPI/PVA nanofiber with SPI/PVAratio of 1:1. (FIG. 22a ) SEM and (FIG. 22b ) and (FIG. 22c ) TEMimages. From these images, one can find that there is no SPInanoparticles formed in the nanofiber, indicating good miscibilitybetween denatured SPI and PVA.

FIGS. 23a-23f . SEM images of the nanofabrics before air filtrationtesting: (FIG. 23a ), pure PVA nanofabric, (FIG. 23b ) SPI/PVA withratio of 0.8:1, (FIG. 23c ) SPI/PVA with ratio of 1.5:1; SEM images ofthe nanofabrics after air filtration testing: (FIG. 23d ), pure PVAnanofabric, (FIG. 23e ) SPI/PVA with ratio of 0.8:1, (FIG. 23f ) SPI/PVAwith ratio of 1.5:1.

FIGS. 24a and 24b . Digital photos of SPI/PVA nanofabrics with highSPI:PVA ratio of 1.5:1. FIG. 24a before filtration testing and FIG. 24bafter filtration testing. There are cracks formed after filtrationtesting, indicating that a high loading of SPI will deteriorate themechanical strength and finally the filtration performance.

FIGS. 25a-25f . SEM images and inserted digital photos of (FIG. 25a )Scott® PT-T, (FIG. 25b ) Scott® PT-P, (FIG. 25c ) Bounty® PT. (FIG. 25d) Air flow resistance (pressure drop) of different paper towels. (FIG.25e ) Particulate removal efficiency of different paper towels vs. PMparticle size. (FIG. 25f ) Particulate filtration efficiency, PM_(2.5)and PM_(10-2.5) for the three paper towel samples.

FIG. 26. Schematics of the preparation for the hybrid filters: proteinnanofiber coating via electrospinning onto the paper towel substrate.The left digital image shows that the nanofiber-coating can be peeledoff from the paper towel and the right digital image shows the foldablehybrid filter. The chemical structures (lower row) of the cellulose andprotein possess rich functional groups.

FIGS. 27a-27h . SEM images of filter samples: pure gelatin nanofabricsand hybrid samples, i.e. gelatin nanofiber-coated PT (G/PT) filter mats:FIG. 27a and FIG. 27c are the back and front surfaces of the gelatinnanofabrics sample before filtration, respectively; FIG. 27b , and FIG.27d are theirs after filtration testing. FIG. 27e and FIG. 27e are theback and front surfaces of the hybrid samples (G/PT) before filtrationtests; FIGS. 27f and 27h are theirs after filtration testing. Inserteddigital photos are the air filter mats (FIG. 27g ) before and (FIG. 27h) after the air filtration testing.

FIGS. 28a-28d . Particulate removal efficiency of various filtersamples: (FIG. 28a ) Particulate removal efficiency vs. various particlesizes (0.3-10 μm); (FIG. 28b ) removal efficiency vs. small PM particlesizes (0.3-2.5 μm); (FIG. 28c ) PM_(2.5) removal efficiency; (FIG. 28d )effect of air flow rate on the particulate removal efficiency of thehybrid G/PT/G filter sample vs. various PM particles sizes.

FIG. 29. Effects of air flow rate on chemical filtration efficiency ofthe hybrid material, G/PT/G filter sample for removal of various toxicgaseous pollutants.

FIGS. 30a and 30b . (FIG. 30a ) Air flow resistance (pressure drop) ofall the filter samples: at standard air face velocity of 5 cm/s (flowrate=4 lit/min); (FIG. 30b ) air flow resistance vs. flow rate of thehybrid G/PT/G filter sample; inserted image is the schematicillustration of pressure drop measurement apparatus.

FIGS. 31a and 31b . SEM images of the hybrid SC/PT filter (FIG. 31a )before and (FIG. 31b ) after filtration testing.

FIGS. 32a and 32b . Air filtration performance of the hybrid filter withsoy protein composite (SC) nanofibers/paper towel (PT) combination:(FIG. 32a ) Particulate removal efficiency vs. PM particle sizes from0.3-10 μm and (FIG. 32b ) toxic chemical removal efficiency for fourgaseous toxic chemicals.

FIGS. 33a-33e . Interaction-based filtration mechanism of the hybridprotein nanofiber-coated PT filters: (FIG. 33a ) schematic illustrationof circulatory flow between PT substrate and nanofiber layers; (FIG. 33b) schematic illustration of interaction-based filtration mechanism forthe hybrid protein nanofiber-coated PT filter mat; (FIG. 33c ) schematicrepresentation of the toxic gas filtration testing apparatus; (FIG. 33d) FTIR characterization of SO₂ gas before and after filtration using PTfilter; (FIG. 33e ) FTIR characterization of SO₂ gas before and afterfiltration using the G/PT filter.

DETAILED DESCRIPTION

Air filters formed from mats of protein-containing nanowires areprovided. The nanowires are formed into a mat with pores that allow airto pass through while physically filtering particulate matter. Theprotein in the protein-containing nanowires also serves to chemicallyfilter polluted air passed through the filter. Specifically, chemicalfunctional groups from the many amino acids that comprise the protein ofthe protein-containing nanowire react with certain chemical pollutants(e.g., carbon monoxide and formaldehyde) in order to capture orotherwise neutralize the pollutant. Accordingly, the single nanofibermat performs two filtering functions. Methods of filtering air using theprovided air filters are also disclosed, as well as methods for makingthe air filters from protein-containing nanofibers.

In one aspect, an air filter is provided. In one embodiment, the airfilter includes a porous nanofiber mat configured to filter particleshaving a diameter of about 0.1 μm or greater when air is passed throughthe air filter, the porous nanofiber mat comprising a plurality ofprotein-containing nanofibers, comprising a protein configured to bindto, and thereby filter, at least one chemical species.

The disclosed air filters provide a new filtration media that can beincorporated into any presently known or future developed air filtrationsystems. Examples include use as the filter in an HVAC system or apersonal breathing mask.

The air filters are formed from a porous nanofiber mat configured tofilter particles having a diameter of about 0.1 μm or greater when airis passed through the air filter. Nanofiber mats in general could havesimilar or smaller pore size in comparison with micron-size fiber mats;however, the average pore size is still in micrometer regime and willnot prevent air from passing through the mat and filter. In a furtherembodiment, the porous nanofiber mat configured to filter particleshaving a diameter of about 0.3 μm or greater. The size of the pores isdefined based on the diameter of the nanofibers and the density of thenanofibers in the mat. The FIGURES include many micrographs ofrepresentative nanofiber mats. FIG. 2c provides an illuminatingperspective of an exemplary nanofiber mat imaged at two differentmagnifications, such that the porous nature of the mat can be clearlyseen.

As used herein, the term “porous” refers to a material containing pores.The skeletal portion of the material is often called the “matrix” or“frame.” The pores are typically filled with a fluid (liquid or gas).The skeletal material is usually a solid, but structures like foams areoften usefully analyzed using concept of porous media. A porous mediumis often characterized by its porosity. Other properties of the medium(e.g., permeability, tensile strength, electrical conductivity) cansometimes be derived from the respective properties of its constituents(solid matrix and fluid) and the media porosity and pores structure, butsuch a derivation is usually complex. Even the concept of porosity isonly straightforward for a poroelastic medium. The concept of porousmedia is used in many areas of applied science and engineering:filtration, mechanics (acoustics, geomechanics, soil mechanics, rockmechanics), engineering (petroleum engineering, bio-remediation,construction engineering), geosciences (hydrogeology, petroleum geology,geophysics), biology and biophysics, material science, etc.

Because particle filtration is one of the two primary functions of theair filter, the pore size is directly related to the efficacy of thefilter's ability to filter particles.

The second function of the air filter is to filter chemical species,such as pollutants. This ability is realized by forming the nanofibermat from a plurality of protein-containing nanofibers, comprising aprotein configured to bind to, and thereby filter, at least one chemicalspecies.

The nanofibers are protein-containing. As used herein, the term“protein-containing” refers to a nanofiber that has at least a portionof protein in its composition. In certain embodiments the entirenanofiber is formed from protein. In one embodiment the entire nanofiberis formed from a single protein. In another embodiment the entirenanofiber is formed from a composite of two or more proteins. In certainembodiments the nanofiber is formed from a composite material thatincludes both a protein and another, non-protein, material, such as apolymer. SPI/PVA nanofiber, disclosed below in the EXAMPLES, is anexample of such a protein-containing material.

As used herein, the term “protein” refers to large biomolecules, ormacromolecules, that include one or more long chains of amino acidresidues. For clarity, a linear chain of amino acid residues is called apolypeptide. A protein contains at least one long polypeptide. Shortpolypeptides, containing less than 20-30 residues, are rarely consideredto be proteins and are commonly called peptides, or sometimesoligopeptides. Proteins differ from one another primarily in theirsequence of amino acids, which is dictated by the nucleotide sequence oftheir genes, and which usually results in protein folding into aspecific three-dimensional structure that determines its activity. Shortproteins can also be synthesized chemically by a family of methods knownas peptide synthesis, which rely on organic synthesis techniques such aschemical ligation to produce peptides in high yield. Chemical synthesisallows for the introduction of non-natural amino acids into polypeptidechains, such as attachment of fluorescent probes to amino acid sidechains. Chemical synthesis is inefficient for polypeptides longer thanabout 300 amino acids, and the synthesized proteins may not readilyassume their native tertiary structure. Most chemical synthesis methodsproceed from C-terminus to N-terminus, opposite the biological reaction.

Importantly, proteins contain chemically active groups that then providechemical functionality to the nanofiber formed using the protein. Thesechemically active groups can then bind with pollutants passing throughthe filter, generating a chemical reaction that captures or otherwisechemically transforms the pollutant from its original state, therebyeliminating the pollutant. Such a feature is particularly desirable inlocations with high chemical pollution, such as Beijing. Specific aspectof the protein-pollutant interaction are discussed in further detailbelow and in the EXAMPLES.

FIG. 1 diagrammatically illustrates the general components and operatingprinciples of the air filter. In particular, a micro/nano-scale diagramof the nanofiber mat is illustrated (“Protein Nanofabrics”); a singlenanofiber is then illustrated; finally, a molecular-scale illustrationof the amino-acids forming the protein nanofiber is provided. As notedin FIG. 1, “Functional groups: interact with toxic chemicals.”Therefore, by tailoring the nanofiber protein composition, and thereforethe amino acid functional group composition, the nanofiber can bedesigned to chemically filter specific species (e.g., pollutants).Carbon monoxide and formaldehyde are two exemplary pollutants that arefiltered in the EXAMPLES (e.g., FIGS. 4a and 4b ).

The plurality of protein-containing nanofibers form the porous nanofibermat that provides filtering capabilities. The mat is made of non-wovenand randomly oriented nanofibers. The configuration and properties ofthe mat may vary based on how the nanofibers are generated and assembledto form the mat. In the exemplary fabrication method disclosed in theEXAMPLES, electrospinning, the formed nanofibers stack up on top of eachother during the process and are bound together via physicalentanglements.

The electrospinning technique is a fiber production method which useselectric force to draw charged threads of polymer solutions or polymermelts up to fiber diameters in the order of some ten nanometers. As thejet dries in flight, the mode of current flow changes from ohmic toconvective as the charge migrates to the surface of the fiber. The jetis then elongated by a whipping process caused by electrostaticrepulsion initiated at small bends in the fiber, until it is finallydeposited on the grounded collector. The elongation and thinning of thefibers lead to the formation of fibers with nanometer-scale diameters.During this process the nanofibers may break and different nanofiberswill stack up on top of each other to form the multi-layer nanofibermat. It is unlikely that a nanofiber mat will be formed viaelectrospinning that is truly a single, extremely long, nanofiber coiledupon itself. However, if such a nanofiber mat were created, it wouldalso be contemplated by the present disclosure.

In one embodiment, at least a portion of the plurality ofprotein-containing nanofibers consist essentially of protein. In thisembodiment, at least some of the nanofibers forming the nanofiber matconsist essentially of protein. An exemplary “all-protein” nanofibermaterial is gelatin, which is described in great detail in EXAMPLE 1. Ina further embodiment, all of the nanofibers of the nanofiber mat consistessentially of protein.

In one embodiment, the protein is selected from the group consisting ofplant-based proteins, animal-based proteins, and synthetic proteins.Representative plant-based proteins include Soy protein isolate (SPI),Canola protein, Zein (corn protein), Seitan (wheat protein), and Gluten(wheat and meat protein).

Representative animal-based proteins include Collagen, Gelatin, Keratin,Casein (mammalian milk protein), Fibrin, Silk, Egg albumen, and wool.

A representative synthetic (artificial) proteins include artificialspider silk.

The protein can be selected based on the particular functional groupsavailable for filtering chemical pollutants. The protein may also beselected based on its ability to form nanofibers having desirablephysical characteristics, such as diameter, length, and stability. As anexample, gelatin can be formed into robust nanofibers on its own(EXAMPLE 1), but SPI cannot be electrospun without forming a compositewith a polymer (EXAMPLE 2). Accordingly, in certain embodiments, atleast a portion of the plurality of protein-containing nanofibers arecomposite nanofibers comprising protein and a polymer. In oneembodiment, the polymer is selected from the group consisting ofpoly(vinyl alcohol) PVA, poly(ethylene oxide) (PEO), poly(acrylonitrile)(PAN), and Nylon. In yet another embodiment, the polymer is selectedfrom the group consisting of PVA and PEO.

In one embodiment, the composite nanofibers have a ratio of protein topolymer, by weight, is in the range of 0.5:1 to 2:1. In one embodiment,the composite nanofibers have a ratio of protein to polymer, by weight,about 1:1.

Turning now to the chemical filtering properties of the nanofiber mat,resulting from the protein-containing nanofibers, in one embodiment theat least one chemical species filtered is selected from the groupconsisting of carbon monoxide (CO), formaldehyde (HCHO), Sulfur oxides(SOx), nitrogen oxides (NOx), Ammonia (NH3), carbon dioxide (CO2),volatile organic chemicals (VOCs), Ozone (O3).

As previously noted, the protein portion can be selected to chemicallyfilter almost any chemical pollutant. As an example, to capture polarchemicals: The protein must contain hydroxyl, carboxyl groups, and/orany type of polar group, such as an amine. Amino acids having theseproperties include lysine, arginine, aspartic acid, glutamic acid,cysteine, glycine, and proline. Protein examples having these groupsinclude gelatin, soy protein, collagen, zein, and gluten.

Conversely, to capture chemicals with aldehyde groups: The proteinshould be rich in amine groups, such as lysine. Exemplary proteins thatare rich in these groups include gelatin, soy protein, collagen, zein,and gluten] Furthermore, the charged groups on amino acids such aslysine and arginine can filter charged pollutants, such as heavy metalions, from air passing through a filter rich in these amino acids.

The EXAMPLES address protein chemical functionality and pollutantfiltering in greater detail.

Turning now to the physical properties of the air filter, mat, andnanofibers, in one embodiment, the air filter is configured for airflowthrough the air filter such that the resistance to air flow is 250 Pa orless at 4 L/min air flow rate. Such a flow rate allows the air filter tooperate similarly to present air filters that do not chemically filterpollutants. The present air filters can be interchanged with commercialfilters and so they can be fabricated to demonstrate the desired airflowcharacteristics when implemented in a filtration system. In a furtherembodiment, the air filter is configured for airflow through the airfilter such that the resistance to air flow is 250 Pa or less at 10L/min air flow rate.

The airflow through the air filter can be defined by the air flowresistance (pressure drop (ΔP)) values. Both systems showed very lowresistance to flow (as an example, ΔP for gelatin air filters=201 Pa;and ΔP for SPI/PVA air filters=215 Pa).

In one embodiment, the porous nanofiber mat has a thickness in the rangeof 8 μm to 30 μm. The nanofiber thickness depends on the areal densityof the nanofiber mat. For example, a 3.43 g/m² gelatin nanofiber filterwould have a thickness of about 16 μm, while a 2 g/m² nanofiber layerwould have a thickness of about 10 μm. Accordingly, in one embodiment,the porous nanofiber mat has a thickness in the range of 10 to 16 μm.

There is no limit on the physical size of the air filters (e.g., width,length, diameter if circular). The nanofiber mat can be cut into anyshape and can be fabricated across a large area (on the order of squarecentimeters or meters). Again, the disclosed air filters are intended asreplacements for any traditional air filter material and therefore canbe fabricated to have a form factor similar to any of the myriad airfilter shapes.

The nanofibers are particularly uniform in diameter, which makes forconsistent performance between similarly manufactured nanofiber mats. Inone embodiment, the plurality of protein-containing nanofibers have anaverage diameter of 1000 nm or smaller. In one embodiment, the pluralityof protein-containing nanofibers have an average diameter of 250 nm orsmaller. In one embodiment, the plurality of protein-containingnanofibers have an average diameter of 200 nm or smaller. In oneembodiment, the plurality of protein-containing nanofibers have anaverage diameter of 150 nm or smaller. In one embodiment, the pluralityof protein-containing nanofibers have an average diameter of 100 nm orsmaller.

In one embodiment, the plurality of protein-containing nanofibersconsist essentially of protein and have an average diameter of 100 nm orsmaller. Examples of such an embodiment are the gelatin nanofiber matsdisclosed in EXAMPLE 1.

In one embodiment, the plurality of protein-containing nanofibers arecomposite nanofibers comprising protein and a polymer and have anaverage diameter of 150 nm or smaller. Examples of such an embodimentare the SPI/PVA nanofiber mats disclosed in EXAMPLE 2.

The protein-containing nanofibers in the porous nanofiber mat aregenerally uniform in diameter size, due to the similar conditions inwhich they are fabricated (e.g., electrospinning). The diameterdistribution is Gaussian. In one embodiment, the plurality ofprotein-containing nanofibers have a Gaussian diameter sizedistribution.

Exemplary analysis of nanofiber size distribution follow.

Average fiber diameter and its standard deviation:

Gelatin: (53 nm-87 nm) have a size distribution of 70 nm±17 nm (69% ofthe fibers have a diameter within this range).

SPI/PVA: (112 nm-160 nm) have a size distribution of 136 nm±24 nm (69%of the fibers have a diameter within this range).

Also, because the distribution function for all systems tested isGaussian, we can use the confidence interval of (3σ) to show theaccuracy of the 99.73% interval. Thus:

Gelatin: (0, 121) or simply: 121 or less 99.73% of the fibers have adiameter within this range.

SPI/PVA: (64, 208) 99.73% of the fibers have a diameter within thisrange.

Substrates for Air Filters

While air filters are free-standing mats of protein-containingnanofibers in certain embodiments, in other embodiments a substrate isused when forming the mats. In this regard, a substrate can providemechanical support for relatively delicate nanofibers and can allow verythin mats to be formed. The substrate must allow air to flow through itat a sufficient rate. The substrate may provide its own particlefiltration properties.

In certain embodiments, the air filter further includes acellulose-fiber layer that is adjacent to or abutting the porousnanofiber mat. In such embodiments, the cellulose-fiber layer is asubstrate for the nanofiber mat. EXAMPLE 3 provides extensive disclosureand testing of such cellulose-fiber (“paper towel”) substrates. In oneembodiment, the cellulose-fiber layer is a paper towel. Commercial andconsumer paper towels can be used as substrates and differentcompositions of paper towels have different properties as a substrate.

In one embodiment, the cellulose fiber layer provides a mechanicalsupport for the porous nanofiber mat and is configured to filterparticles from the air passed through the air filter. The cellulosefiber layer itself can filter out the particles of as small as 0.3 μmdiameter with efficiency ranging from 9% to 45%, depending on the typeof paper towel, as set forth in EXAMPLE 3.

When a substrate is used, a single nanofiber mat can be applied to oneside of the substrate. In further embodiments, however, a second porousnanofiber mat is disposed on the opposite side of the substrate.Accordingly, in one embedment, when the substrate is a cellulose fiberlayer, the air filter includes a first porous nanofiber mat on a firstside of the cellulose fiber layer and a second porous nanofiber mat on asecond side of the cellulose fiber layer, opposite the first side.

In one embodiment, the second porous nanofiber mat is adjacent (close tobut not necessarily touching—could be separated by an intermediatelayer) or abutting (touching) the cellulose-fiber layer on an oppositeside in relation to the porous nanofiber mat.

In one embodiment, the composition and configuration of the secondporous nanofiber mat are the same as the porous nanofiber mat.

In one embodiment, the composition or configuration of the second porousnanofiber mat is different than the porous nanofiber mat.

Methods of Filtering Air

In another aspect, a method of filtering air is provided. In oneembodiment, the method includes passing air through an air filter asprovided herein. As noted previously, the disclosed air filters formedfrom porous nanofiber mats are intended to replace present air filtersin essentially any application. Accordingly, the method of filtering airbroadly include any application of the disclosed air filters forfiltering air, wherein the air includes particulate matter and chemicalpollutants, both of which are filtered (at least partially) by theprotein-containing nanofibers.

In one embodiment, the step of passing air comprises forcing air throughthe air filter. In one embodiment, the air filter is incorporated intoan air filtration system. Representative air filtration systems includeHVAC systems, personal masks, residential, automotive industries,hospitals filtration, etc.

Methods of Making the Porous Nanofiber Mats

In another aspect, a method of making an air filter according to thedisclosed embodiments is provided. In one embodiment, the methodincludes electrospinning a solution comprising a solvent and a precursorto a protein-containing nanofiber. Electrospinning is a well-knowntechnique for generating nanofibers on a substrate. Electrospinning isapplied in the present embodiments to solutions that formprotein-containing nanofibers. Particular examples of the methods areincluded in the EXAMPLES, wherein gelatin nanofiber mats are formed inEXAMPLE 1, SPI/PVA (protein/polymer composite) nanofiber mats are formedin EXAMPLE 2, and cellulose fiber substrate (“paper towel”) air filtersare formed in EXAMPLE 3.

An exemplary method of making a gelatin nanofiber mat viaelectrospinning is as follows:

-   -   Gelatin powder is dissolved in 80% (v/v) aqueous acetic acid at        65° C. to achieve a 18 wt % gelatin solution    -   The gelatin solution was loaded in a syringe with a 21-gauge        blunt-tip needle    -   An operating voltage of 18-20 kV was employed for the        electrospinning and was controlled by a high voltage power        source    -   The distance between needle and sample collector was fixed to be        10 cm and an average flow rate of 0.6 ml/h was utilized    -   Commercial aluminum mesh with wire diameter of 0.011 inch and        mesh size of 18×16 was grounded to collect the gelatin fibers.

An exemplary SPI/PVA nanofiber mat fabrication method is as follows:

-   -   Soy protein isolate was thermally denatured in mixed solvent        (volume ratio, acetic acid:DI water=80:20) with a concentration        of 8.5 wt % at 85° C. for 4 h using magnetic stirring (400 rpm)    -   Poly(vinyl alcohol) was dissolved separately in the same solvent        with a concentration of 8.5 wt % at 60° C. for 2 h using        magnetic stirring (400 rpm)    -   The denatured SPI was loaded as a solution into the PVA solution        with different ratios and mixed with PVA solution for 24 h using        a spin mixer    -   The SPI/PVA nanocomposite solution was loaded in a syringe with        a 21-gauge blunt tip needle    -   A voltage of 16-21 kV was applied for electrospinning and was        controlled by a high voltage power source.    -   The distance between needle and sample collector was fixed to 10        cm and average flow rate of 0.6 ml h-1 was utilized    -   Commercial aluminum mesh with wire diameter of 0.011 inch and        mesh pore size of 1 mm×1 mm was grounded to collect the fibers.

An exemplary protein nanofiber-coated paper towel filter mat fabricationmethod is as follows:

-   -   Gelatin and SPI/PVA solution are prepared following the steps        shown above    -   A paper towel substrate is fixed on a conductive copper mesh        collector and the nanofibers are deposited on the paper towel        substrate with desired fiber areal density    -   The operating voltage, spinning flow rate, and        needle-to-collector distance are adjusted based on the solution        (gelatin or SPI/PVA).

As used herein, the term “about” indicates that the subject number canbe modified by plus or minus 5% and still fall within the scope of thedisclosed embodiment.

The following EXAMPLES are included for the purpose of illustrating, notlimiting, the disclosed embodiments.

Example 1: Gelatin Nanofiber Filters

Particulate and chemical pollutants are ubiquitous in polluted air.However, current air filters using traditional polymers can only removeparticles from the polluted air without incorporating additional activematerials. To efficiently filter both particulates and chemicalpollutants, development of environmentally friendly air filter materialsis in critical need. In this study, gelatin is employed as an example tostudy the potential of natural proteins as high-performanceair-filtering material. Based on optimized composition of a “green”solvent, uniform gelatin nanofiber mats with small diameters werefabricated via electrospinning approach. It is found that the resultingnanofabrics possess extremely high removal efficiencies for bothparticle matter (with a broad range of size from 0.1 μm to 10 μm) andvarious toxic chemicals (e.g. HCHO and CO). Moreover, these efficienciesare realized from the protein nanofabrics with a much lower arealdensity (3.43 g/m²) compared with that of commercial air filters (e.g.164 g/m² for high efficiency particulate air filter (HEPA)). This studyreveals that nanofabrics of natural proteins hold great potential forapplication in “green” and multi-functional air filtering materials.

1. Introduction

Air pollution has been a great concern in big cities recently. Therelease of chemicals, particulate and biological materials into air canlead to various diseases or discomfort to humans and other livingorganisms, alongside their impacts on the environment. The combinationof particles and chemical pollutants can make the polluted air even moreharmful. Particle Matter (PM) is usually categorized into two groups,PM_(2.5) and PM_(10-2.5) which denote particles with aerodynamicdiameters smaller than 2.5 μm and between 2.5-10 respectively. Accordingto the 2009 and 2012 World Bank report, more than 60% of Americans livein air quality levels that are potentially detrimental to health. Recentstudies have reported a more serious PM pollution problem in developingcountries. A high degree of air pollution was responsible for numerouspremature deaths. PM_(2.5) particles are the critical particulatepollution to be filtered due to their ability to penetrate into humanlungs and bronchi. Indoor air quality has become an increasing issue aswell. More and more buildings incorporate air filtration protection intheir heating, ventilation, and air conditioning systems, but asignificant amount of energy is required to maintain the air exchangeprocess due to a high air-resistance (pressure drop) of the air-filters.Therefore, air filters with high-efficiency of removing particles andchemicals simultaneously are in critical need.

Understanding of the composition of polluted-air is critical for thedevelopment of air-filtering materials. In general, the composition ofpollutants in polluted-air is extremely complicated due to thecomplexity of the sources of pollution. PM particles can be producedfrom variety of sources, such as fuel combustion in vehicles, industrialfactory plants, cigarette smoke, dust, etc. These PM particles behavedistinctly due to their diverse chemical composition. Most PM_(2.5)particles are composed of organic compounds such as carbon-derivedmatters (e.g. carbon dioxide and carbon monoxide), inorganic compounds(e.g. sulfur dioxide (SO₂ ²⁻), sulfate (SO₄ ²⁻), silicon dioxide (SiO₂),and nitrate (NO³⁻), etc.), and biological threats (e.g. bacteria andviruses). These particles are very stable in air and have lifetimesbetween hours to weeks due to their very small sizes. They can scattervisible light and reduce visibility because of the similarity betweentheir particle size and visible light wavelengths. In addition to PMparticles, polluted air includes a wide variety of chemical gases suchas carbon monoxide (CO), nitrogen dioxide (NO₂), methane (CH₄), benzene,dioxin, ozone, etc. A large number of chemicals in polluted air areclassified as volatile organic compounds (VOCs) which are primarilyemitted by petrochemical and allied industries. VOCs can undergodifferent kinds of photochemical reactions in the atmosphere and causevarious environmental hazards. In gas phase carcinogenic or otherwisetoxic VOCs present a danger to humans. Since the polluted air is usuallycomposed of pollutants with complicated compositions and physicochemicalproperties, multi-functional air filtering materials that are able togenerate various types of interactions with the pollutions are of greatinterest for air-filtration applications.

Air filters are the most commonly used devices to remove pollutants fromthe air. They have been widely used in different areas, e.g. automotiveindustries, residential, general commercial, and even hospitals, generalsurgeries and so on. The filtration function is mainly realized viaphysical and PM size-based capturing mechanisms. There are four primarymechanisms for filtration based on the size of the pollutant particles.Sieving is one of the most important mechanisms and is only effectivefor particles with sizes larger than the pore size of the filter. Forparticles with sizes smaller than the pore size of the filter, inertialimpaction, interception, and diffusion are the dominant mechanisms forfiltration. In specific, interception occurs when small particles flowwith the air stream and come into contact with the fiber surface. Theattractive interactions between the small particles and fibers play acritical role for this mechanism. The diffusion mechanism is effectivefor even smaller particles with aerodynamic size smaller than 100 nm.For these particles, Brownian motion dominates movement and capturingoccurs via random collision. Traditionally, air filters are made ofporous films, such as non-woven fibrous mats with randomly orientedmicron-size fibers. These types of air-filtering materials have severaldisadvantages as explained below. First, the fibers are made ofchemically synthesized or petroleum based materials, such aspolypropylene and fiberglass. These conventional materials provide verylimited chemical functionality, resulting in insufficient interactionswith pollutants. Secondly, disposing of used air filters made of thesematerials can cause further environmental pollution as most of them arenot environmentally friendly. Finally, microfiber-based air filteringmaterials possess limited surface area, which further deteriorate thefiltration performance.

To address the above issues related to conventional air-filtermaterials, nanofiber mats have been of great interest recently.Nanofiber mats possess several advantages as explained below. Firstly,nanofibers will tend to absorb substance from the environment due to ahigh surface energy, which enhances the interactions between fibers andpollutants. Secondly, nanofibers can significantly increase the surfacearea of filter materials. In other words, nanofibers provide more activesites for trapping pollutants. As a result, nanofiber mats can realizehigh filtration efficiency for PM while possessing low-pressure drop orair resistance, which is critical for their practical application. As aresult, nanofabrics of polymers rich in functional groups represent apromising solution for high-performance air-filtering materials. Inparticular, biomaterials, such as natural proteins, are promisingcandidates as high-performance air filtering materials. It is well knownthat proteins are rich in functional groups, that is, the R-groups onthe amino acids. These functional groups make proteins an ideal materialfor air filtering applications. For example, Chitosan has been mixedwith poly(ethylene oxide) and fabricated into nanofibers as an airfilter material. The cationic nature of chitosan was used to achievemore than 70% removal of heavy metal ions and aerosol particles from theair. Other biomaterials were also studied as air filter materials,however, they were usually mixed with conventional polymers to fabricatenanofibers. As a result, the potential of pure protein nanofabrics ashigh-performance air filtering materials has never been studied based onthe author's knowledge.

In this study, the potential of pure protein nanofabrics for airfiltering application is investigated. It is believed that thecombination of nanomaterials with natural proteins can lead to apowerful nanofabric with the ability to trap various kinds ofpollutants, including particulate and toxic gas. In particular, gelatinis employed as an example for that. Gelatin protein is derived fromthermal denaturation of collagen, the most abundant protein in human andanimal bodies. Fabrication of gelatin nanofibers has been proved verysuccessful and they are usually reported as scaffolds for food, energy,pharmaceutical, environmental, and medical applications, except as airfiltering material. Here, to study gelatin nanofabrics for airfiltration purposes, the fabrication of gelatin nanofibers is furtherimproved. Firstly, instead of using toxic solvents (e.g.2,2,2-trifluoroethanol (TFE) or 1,1,1,3,3,3-hexaflouro-2-propanol(HFIP)) which are usually used for the electrospinning of gelatin, anon-toxic solvent (mixture of acetic acid and water) is employed.Secondly, the diameter of the gelatin nanofibers is further reduced tobe around 70 nm, which is smaller than the typical values (ca. 100 nm)for gelatin nanofibers.

2. Materials and Methods

2.1. Raw Materials and Solution Preparation.

Gelatin powder (Type A) produced from porcine skin was supplied fromSigma-Aldrich (MO, USA). Acetic acid (99.9% purity) was purchased fromJ.T.Baker® (PA, USA). Gelatin was dissolved in mixed solvent (volumeratio, acetic acid:DI water=80:20) with a concentration of 18 wt % at65° C. The mixed solvent was used to achieve a good electrospinning ofthe gelatin solution. With that ratio between water and acetic acid, itwas found that a homogenous yellow solution and stable electrospinningof the solution can be achieved.

2.2. Preparation of Protein Filter Nanofabrics.

Protein nanofabrics were prepared by electrospinning techniques. Thegelatin solution was loaded in a syringe (Monojet™ Kendall) with a21-gauge blunt-tip needle. An operating voltage of 18-20 kV was employedfor the electrospinning and was controlled by a high voltage powersource (ES50P-5W, Gamma High Voltage Research). A mono-inject syringepump (KD Scientific, KDS-100) was utilized to pump the gelatin solution.Commercial aluminum mesh with wire diameter of 0.011 inch and mesh sizeof 18×16 was grounded to collect the gelatin fibers. The distancebetween needle and sample collector was fixed to be 10 cm and an averageflow rate of 0.6 ml/h was utilized.

2.3. Polluted-Air Samples Preparation and Air-Filtration Testing.

Cigarette smoke and the product of burning plant materials were used asthe sources of pollution to prepare polluted-air samples. It has beenestimated that cigarette smoke includes PM particles with a broad rangeof sizes (0.01 to 10 μm), and more than 7000 different chemicals,hundreds of which are toxic. The most dangerous chemicals of interestfor filtration are carcinogens, such as formaldehyde (HCHO), carbonmonoxide (CO), ammonia (NH₃), hydrogen cyanide (HCN) and toxic metalions (chromium (Cr³⁺, Cr⁶⁺), cadmium (Cd²⁺), lead (Pb²⁺). The product ofburning plant materials also consists of similar types of pollutantswith varying concentrations. Since the original polluted-air sampleswere so concentrated with PM and chemicals, they were diluted in a gasbag to a hazardous level which can be measured by the analyzer. Thediluted polluted-air with detectable levels of pollution was used as thefinal polluted-air sample for air-filtration testing. Before theair-filtration testing, the initial concentrations of PM with differentparticle sizes (0.3-10 μm) and toxic chemicals (HCHO and CO) in the airsamples were measured by a particle counter (CEM, DT-9881). To performthe air-filtration testing, the pressure difference of both sides of airfilter was controlled and measured by a manometer (UEi, EM201-B) with astandard air flow velocity of 5 cm/s to investigate the air flowresistance of the air filter material. In all the measurements, acircular filter sample with diameter of 37 mm was placed in a home-madesample holder. The filtered air was collected by another clean gas bagwhich was vacuumed in advance. When the air-filtration testing ended atdifferent filtering time, the concentrations of the PM and toxicchemicals inside the clean gas bag with filtered air were measured andrecorded. Via the equation (1), one can determine the removal efficiencyη_(p).

η_(p)=(C _(p) −C _(c))/C _(p)  (1)

where C_(p) is the concentration of the pollution in the polluted-airsample before air filtration testing, and C_(c) is the concentration ofthe pollution in the filtered air sample.

2.4. Characterizations.

To study how the particle pollutions were removed by the proteinnanofabrics, SEM (FEI SEM Quanta 200F) was employed to investigate themorphology of the protein nanofabrics before and after air filtrationtesting. The samples were sputter-coated with 10 nm gold nanolayer inthickness using Technics Hummer V sputter coater. In order to study thepossible interactions between the protein nanofabrics and pollutants,FTIR (Nicolet, Thermo Scientific) absorption spectra was employed. Todistinguish the interactions between nanofibers and pollutants from theinteractions inside the fabric or polluted-air themselves, the FTIRspectrum of three kinds of samples were recorded and compared. Thesesamples include polluted-air, clean protein nanofabrics before and afterfiltration. All the measurement was repeated at least 3 times and goodrepeatability was found for these samples.

3. Results and Discussion

Gelatin Nanofabrics/Nanofiber Mats.

The target of this work is to study the potential of gelatin nanofabricsas high-performance air filtering material with two levels of airfiltering functions: (1) removing particles, such as dust, pollen, withparticle sizes in a broad range from 0.1 to 100 μm; (2) removing toxicor obnoxious gases, such as formaldehyde and carbon monoxide in tobaccosmoke. It is known that gelatin molecules possess a broad range offunctional groups in their multi-level structures. The characteristicsof these chemical structures provide capability for interaction withmultiple species of polar molecules, which lead to a great potential tocapture many chemicals. Specifically, gelatin consists of glycine(21.4%), proline (12.4%), hydroxyproline (11.9%), and glutamic acid(10.0%) in its amino acid profile. The amino acids bring gelatin variousfunctional groups (such as carboxylic and hydroxyl, charged groups, andmany other polar/nonpolar functional groups). These functional groupscan act as active sites generating numerous interactions withpollutants, including hydrogen bonding, ionic bonding, and charge-chargeinteractions and so on. Combined with electrospinning technique, gelatinnanofibers can be fabricated (FIG. 1). Electrospinning is an effectivemethod for making uniform nanofibers with high aspect ratio, and highpore interconnectivity with size ranging from micron to nanometer scale.

First of all, an appropriate solvent, in particular a non-toxic solvent,needs to be selected for preparing the gelatin solution effective formaking nanofibers via electrospinning. Many studies have been reportedon the fabrication of gelatin fibers by using toxic solvents (TFE,HFIP), which yielded average fiber diameter ranges from 100-600 nm. Inthis study, gelatin nanofabrics were fabricated by employing aqueousacetic acid (AA) as a “green” solvent. In order to achieve efficientmolecular dissolution of gelatin, good electrospinability and, as theresult, uniform filter mat with nanoscale fiber diameter, the solventcomposition must be adjusted. More importantly, the uniformity of thenanofibers in the mat along with smaller fiber diameters can result inhigh surface area for capturing more pollutants, which enables thefilter to achieve high filtration efficiencies. Therefore, the effect ofAA-to-water ratio on the resulting nanofiber diameters and theirdistribution was studied. The mixture solvent with optimized compositionwas determined for spinning out uniform gelatin nanofiber mats. Themicrostructures of gelatin nanofibers prepared in different ratios ofthe solvent and the fiber morphology of a commercial HEPA filter werecompared using scanning electron microscopy (SEM) images (FIGS. 2a-2d ).It can be seen that by increasing the ratio of AA to water in thesolvent from 60:40 to 80:20, the size of nanofibers was reduced from ca.470 to ca. 70 nm. Moreover, the nanofiber uniformity was improvedsignificantly: the relative deviation of the nanofibers reduced from 53%to 25% (see fiber diameter distributions and inserted table showing thestatistic results in FIGS. 2a-2d ). These results show that by using themixed solvent with the AA-to-water ratio of 80:20, uniform gelatinfilter mats with the nanofiber diameter of 70 nm were successfullyfabricated, which is smaller than the reported studies showing the fiberdiameter of few hundred nanometers. Reduction in the nanofiber diametercan significantly improve the air filtration capabilities for bothparticulate and toxic chemicals due to their high active surface areas.In general, nanofibers with high surface-to-volume ratio and surfaceactivity will have high particle removal efficiency via interception,diffusion, and other mechanisms while retaining very low resistance toair flow which results in low pressure drop. At the same time, due tothe high density of functional groups along the gelatin nanofibers, thenanofiber mats are expected to possess multiple filtering functions:toxic chemical and particulate filtration. The following air filteringperformance studies were focused on the gelatin filter nanofibersproduced using the optimal solvent.

Particulate Filtration.

First, the morphology of gelatin nanofibers were studied via SEM and theresults are shown by FIGS. 3a-f . In specific, FIGS. 3a and 3d (also seeFIG. 7a-c , which are the SEM images of the nanofabrics before airfiltration testing, that is, the morphology of as-spun pure gelatinnanofabrics. FIGS. 3b and 3d-3f (also see FIGS. 7d-7f are the SEM imagesof the nanofabrics after air filtration testing. The digital photosinserted in the SEM images are the nano-filter before and afterfiltration. The obvious color change from white to yellowish indicatesthat the nano-filter has absorbed lots of pollutants. Further, the SEMimages confirm this point as one can observe lots of particles have beentrapped on the surface of gelatin nanofibers. The notable change incolor should also be related to the absorption of some toxic gases,which will be discussed later. Moreover, the particulate air filtrationefficiency results are shown in FIGS. 3g and 3h . It is found that, forthe gelatin nanofabrics, the removal efficiency is dependent on the PMsize as well as the areal density of the nanofabrics (shown in FIG. 3g). It can be found that with a high areal density (e.g. greater than ca.3.43 g/m²), the gelatin nanofabrics showed almost similar removalefficiency of above 99.20% for PM with sizes from 0.3 to 10 μm. Inparticular, the removal efficiency for the most penetrating particlesize (MPPS) of 0.3 μm particles was significantly improved from 77.10 to99.32% by increasing the areal density of the nanofabrics from 2.25 to3.43 g/m². PM particles with size around 0.3 μm (PM_(0.3)) are known asthe hardest to capture and a high-performance air filter should giverise to an efficiency above 95% for PM_(0.3). The results indicate thatthe areal density of the nanofabrics is critical for the removalefficiency of small particles, but not large particles. This result canbe explained as the difference in the mechanisms for filtering largeparticles (sieving) and small particles (smaller than the pore size).Specifically, large particles were removed by size effects, while smallparticles were trapped via the strong interactions between particles andnanofibers. The removal efficiency for PM_(2.5) and PM_(10-2.5) viananofabrics with different areal density is shown in FIG. 3h . Inparticular, the areal density of 3.43 g/m² is close to an optimal(minimum) value to achieve the highest removal efficiency of bothPM_(2.5) (99.51±0.23%) and PM_(10-2.5) (99.63±0.11%). For the filterswith areal densities higher than 3.43 g/m², the removal efficiency ofboth PM_(2.5) and PM_(10-2.5) was almost constant. The gelatinnanofabrics are promising air filtering materials with removingefficiency for PM_(2.5) higher than 95%, the standard suggested forhigh-efficiency air filters. More significantly, the gelatin nanofabricscan achieve the high efficiency for PM by a much lower area density (ca.3.43 g/m²) as compared with the most successful commercial one (HEPA,164 g/m²). The high removal efficiency for PM is likely contributed bythe combination of “nano-size” effects and the surface properties ofgelatin nanofibers. The diversity of functional groups on the nanofibersurface can provide strong interactions with the pollutants, which iscritical for the removing of PM with sizes much smaller than the poresize of the nanofabrics and of toxic gases as will be discussed later.

Toxic Gases Filtration.

Removing of toxic chemicals via air filters with high efficiency ischallenging since they are small molecules with sizes much smaller thanthat of particles. Conventional ways to remove toxic gases are usingabsorptive particles with high specific surface area, such as activatedcarbon. For the gelatin nanofabrics, two kinds of toxic chemicals,formaldehyde (HCHO) and carbon monoxide (CO) which can be detected bythe analyzer, were chosen as examples to test the chemical removalcapability. FIG. 4a shows the chemical removal efficiency offormaldehyde for gelatin nanofabrics with different areal densitiescompared with that of commercial HEPA filter. It can be seen that thechemical capturing efficiency of formaldehyde increases from 65.0% to83.0% by increasing the areal density of the filter from 2.25 to 3.80g/m². Moreover, the removal efficiency of carbon monoxide as shown inFIG. 4b increased from 62.3% to 76.1% for gelatin nanofabrics with thesame change in areal density. In comparison, for the commercial HEPAfilter, one of the most successful air filter used today, the chemicalremoval efficiencies of formaldehyde and carbon monoxide molecules areless than 5 and 3%, respectively. The high chemical removal efficiencyindicates that the combination of nanofabrics with functional polymersis the key to remove toxic chemicals which cannot be removed bysize-based mechanisms. Therefore, protein nanofibers provide a promisingsolution for multi-functional air filtering materials.

Pressure Drop and Figure of Merit (Quality Factor).

In addition to the particulate and chemical removal efficiency, air flowresistance (pressure drop) is another critical parameter describing theperformance of an air filter (schematic of pressure drop measurementsetup is inserted in FIG. 4c ). High pressure drop downstream of an airfilter will consume a large amount of energy due to the pumping requiredto provide a sufficient air flow, which makes the air filter energyconsuming. The suggested range for pressure drop (ΔP) set by DOE is lessthan 1.3 in.H₂O (approximately 320 Pa). The effect of areal density onpressure drop of the gelatin filters was investigated using standard 5cm/s air face velocity. FIG. 4c shows that the air flow resistance ofthe filters increases with the areal density. Quantitative analysisdemonstrated that the pressure drop of the gelatin nanofabrics with thelowest areal density (2.25 g/m²) was ca. 143 Pa and increased toapproximately 201 Pa for the filter with the highestparticulate/chemical efficiency (3.43 g/m²). These pressure drop valuesmeet the requirement of a high-efficiency filter. The pressure dropalong with removal efficiency allow us to determine the optimal arealdensity of the gelatin nanofabrics for achieving a good balance betweenhigh removal efficiency of pollutants (PM and toxic chemicals) andpressure drop, which is critical for practical applications. For thegelatin nanofabrics when the areal density is higher than ca. 3.43 g/m²,such as 3.80 and 7.67 g/m², the removal efficiency is improved by lessthan 0.5% (see FIG. 3h ), while the pressure drop increases by more than74% (see FIG. 4c ). Thus, 3.43 g/m² should be close to the optimal arealdensity for the gelatin nanofabrics. To comprehensively describe thefiltration performance, the pressure drop and removal efficiency iscombined into one parameter, the quality factor (QF), also known asfigure of merit (FOM), which can be calculated using Equation (2).

QF=−ln(1−η_(p))/ΔP  (2)

where η_(p) is the removal efficiency and ΔP is the correspondingpressure drop. QF is a representative of the ratio between removalefficiency and the air flow pressure drop. This quantitative factorindicates that a good air filter should provide a high removalefficiency and a low pressure drop; hence, a higher QF means a betterfiltration performance of an air filtering material. FIG. 4d shows thecomparison of figure of merit of the gelatin filter fabrics withcommercial air filters and poly(acrylonitrile) (PAN-85) nano-filters asreported recently. It can be found that the gelatin nanofabrics with3.43 g/m² areal density possess the highest quality factor (0.026 Pa⁻¹)among them. The quality factor for gelatin nanofabrics was calculated at30 minutes of filtration testing and 5 cm/s. It is noted that the figureof merit of an air filter (including the removal efficiency and pressuredrop) is never constant and will change with time of using in practice.

Stages of Filtration Process.

For air filtering materials, analysis of the pollutant absorptionprocess is critical for understanding the long-term filtrationperformance. For the multi-functional gelatin air filtering material,the pollutant absorption process was tracked quantitatively viacalculation of the pollutant weight-percentage gain and pollutant weightabsorption rate over time using Equation (3) and (4).

W _(p)=(W _(t) −W _(f))/W _(f)  (3)

W _(p) _(_) _(rate) =ΔW _(pollutant) /Δt  (4)

where W_(p) is the pollutant weight percentage gain, W_(t) is the weightof the air filter after filtration of time t, W_(f) is the weight of thepure air filter before filtration test, W_(p) _(_) _(rate) is thepollutant weight absorption rate, and ΔW_(pollutant) is the absoluteweight of the pollutants absorbed between each time interval. Resultsare compared with those of the commercial HEPA filter in FIGS. 5a and 5b.

At the early stages of filtration (first 30 minutes to 1 hour),particles migrate and merge to form bigger, spherical aggregates.Moreover, the particle weight percentage gain reaches 53% of thenanofabrics' weight and sharply increases after 1 hour of testing to106% while the HEPA filter only showed an increase from 1% to 1.3% dueto its very high areal density (shown in FIG. 5a ). With the increase offiltering time (after 2 hours), more particles were trapped by thenanofibers. The accumulation of particles is also coupled with complexdeformation/transformation processes due to possible physicochemicalinteractions. This process has also been reported in the recent study ontransparent PAN air filters. The phenomenon results in a decrease ofclear filter area. In this stage, the particle weight percentage gainfor gelatin nanofabrics increased moderately to 130%, while theabsorption rate of pollutants decreased significantly (shown in FIG. 5b), which can prove the hypothesis mentioned above. By further increasingthe filtration time, the particle weight percentage gain of the filterreached a plateau region (gelatin nanofabrics and commercial HEPA filterreached to 150% and 2.3% pollutant weight percentage gain, respectively)and the pollutant absorption rate was decreased even more significantlyfor both filters. However, the pollutant absorption rate of gelatinnanofabrics was higher than that of commercial HEPA filter due thefunctional and active surface of gelatin nanofabrics which enable it toabsorb more particles and chemicals within a shorter time period. Thepollutant absorption phenomenon is critical for practical applicationsas it is related to the long-term performance or the life-time of theair filter material. Also see FIGS. 9a-9c for SEM images showing theevolution of surface morphology of gelatin nanofabrics during filtrationof smoke.

Filtration Mechanism Analysis.

To further analyze the performance of the gelatin nanofabrics, thefiltration mechanisms were studied based on examining the surfacechemistry of pollutants and gelatin nanofabrics before and afterfiltration. As mentioned above, numerous functional groups exist in thestructure of gelatin. These functional groups can strongly interact withvarious pollutants in the air and enable the filter to remove thepollutants (toxic chemicals and solid particles) via aninteraction-based mechanism besides existing size-based primarymechanisms. FIG. 6a depicts a simplified gelatin molecule (filter) alongwith PM, and formaldehyde as examples of solid and gaseous pollutants.It can be seen that the aldehyde group can undergo addition reactionswith amine groups in gelatin (provided by amino acids such as lysine)forming aldimine linkages. This reaction elicits a change in filtercolor from white to a yellow color. PM particles and other pollutantswith different compositions can interact with the gelatin nanofabricsthrough hydrogen bonding, charge-charge interactions etc. FIG. 6b showsan SEM image of gelatin nanofilter after the capture of pollutants via acombination of interaction-based and size-based mechanisms. Thisschematic indicates a new interaction-based mechanism of filtration forgelatin nanofabrics besides the primary four size-based mechanisms. Tofurther understand the possible interactions between the gelatinnanofabrics and the pollutants (solid particles and toxic chemicals),Fourier Transform Infrared Spectroscopy (FTIR) and dielectric constantmeasurement (see FIGS. 8a and 8 b) were used to identify the functionalgroups from the pure gelatin nanofabrics, polluted air, and nanofabricswith trapped pollutants. The FTIR spectra of the pollutant in thepolluted-air sample is shown in FIG. 6c . The specific peaks of thefunctional groups in the cigarette smoke are around 3,638, 2,357, 1,659,1,552, 1,501, and 1,458 cm⁻¹ which indicate the existence of O—H, C—H(aldehyde), C═O, and C—O (last three peaks) groups, respectively. All ofthese groups in polluted-air sample may interact with the groups on thesurface of gelatin nanofibers. The comparison of the FTIR spectra forthe gelatin nanofibers before and after filtration is shown in FIG. 6d .It can be found that there is no new peak formed after filtration.However, there is a significant change in the intensity at specificgroups and interactions, including hydroxyl, carboxyl, and aminefunctional groups (see FIG. 6e ). These results can be explained bypositing that the types of interactions between the pollutants in thesmoke and gelatin fibers are similar to those inherently existing ingelatin nanofabrics. Therefore, the dramatic increase in the peakintensity of these functional groups after filtration testing should bethe result of the interactions between gelatin nanofabrics and thepollutants, such as hydrogen bonding, ionic bonding, charge-chargeinteractions, etc.

The dielectric measurements (FIG. 8a ) demonstrated that the commercialHEPA filter, which has no active functional groups in its structure,showed a constant dielectric constant which means, as expected, it is anisolating material. In contrary, gelatin nanofabrics showed higherdielectric values at lower frequencies which means that the gelatinfibers do not have an isolating behavior. In addition, fluctuations inthe permittivity values (FIG. 8b ) of a material at low frequencies aremostly representative of the rotation and the respond of activefunctional groups with the electric field to some extent. These resultsshowed a huge amount of fluctuations at lower frequencies for gelatinnanofibers while there was not any changes for commercial HEPA filterthat is made of an isolating material. Dielectric test results, as wellas FTIR results that have been mentioned previously, proved theexistence of many active functional sites to interact with particles andchemicals present in polluted air that resulted in high removalefficiency for both PM_(2.5) and toxic chemicals (HCHO and CO) with verylow areal density compared with commercial HEPA filter.

Conclusion

In summary, the gelatin protein was studied as an example to demonstratethe potential of natural proteins to serve as environmentally friendlyand high-performance air-filtering materials. Uniform gelatin nanofibermats with very small diameters were fabricated by employing a “green”solvent with optimized composition. It has been found that the gelatinnanofiber mats with a controlled uniformity and small fiber diameterspossess extremely high particulate removal efficiencies of more than99.3% and 99.6% for PM0.3 and PM_(2.5), respectively. These resultsindicate that the gelatin nanofibers with a much lower areal density(e.g. 3.43 g/m²) can efficiently remove a broad range of PM particlessimilar to one of the most efficient particulate air filters, HEPA withareal density of 164 g/m². More significantly, the combination of theinherent surface chemistry of gelatin nanofibers (i.e., variousfunctional groups on the fiber surface) and nanofiber technology enablesgelatin protein nanofibers to have high interaction capability withtoxic chemicals present in the air. Particularly, the gelatinnanofabrics possess excellent efficiency of absorbing toxic chemicals(e.g. ca. 80% for HCHO; 76% for CO), which has never been realized inany air filters with a single material composition. The mechanismsresponsible for such simultaneous high capturing capabilities ofparticulate and toxic chemical were analyzed. It is believed that theinteraction-based filtration mechanism besides the existing size-basedprimary mechanisms result in these functions. This study indicates thatprotein nanofabrics are promising “green” air-filtering materials fornext generation air filtration systems.

Example 2: Soy-Protein Based Nanofiber Filters

Proteins are well-known by their numerous active functional groups alongthe polypeptide chain. The variety of functional groups of proteinsprovides a great potential for proteins to interact with airbornepollutants with varying surface properties. In this work, soy protein, atype of abundant plant protein, has been employed for the first time tofabricate multifunctional air filtration materials. To take advantage ofthe functional groups of soy protein for air filtration application, thesoy protein is first well denatured to unfold the polypeptide chains andthen fabricated into nanofibers with the help of poly(vinyl alcohol). Itis found that the resultant nanofabrics show high filtration efficiencyfor not only airborne particulates with a broad range of size, but alsovarious toxic gaseous chemicals (e.g. formaldehyde and carbon monoxideas demonstrated here), which has not been realized by conventional airfiltering materials. This study indicates that protein-based nanofabricsare promising nanomaterials for multifunctional air-filtration

Introduction

Air pollution has been a growing concern and the cleaning of pollutedair becomes more and more challenging mainly due to a complicatedcomposition of the pollutions, containing particulate matter (PM) withvarious sizes, chemical vapors, and bacteria/virus and so on. Thesecomplicated air pollutants present discomfort and serious impact tohuman health and other living organisms. Most of the particle pollutionsare made of organic compounds, such as carbon derived species (e.g. CO₂and CO) as well as sulfur and nitrogen based inorganic compounds (e.g.SO₂ ²⁻, SO₄ ²⁻, NO₃ ⁻, etc.). Additionally, there are also various toxicgaseous molecules, such as nitrogen dioxide (NO₂), methane (CH₄), carbonmonoxide (CO), formaldehyde (HCHO), and in general volatile organiccompounds (VOCs). These pollutants produced from different sources (suchas petrochemical and allied industries) can participate in variousphotochemical reactions in the atmosphere and create huge amount ofenvironmental hazards. At the same time, these particles and chemicalscan form various derivant pollutants which can easily penetrate intohuman lung and bronchi and cause numerous premature deaths. Therefore,the demand for high-efficiency air filtering materials that are able tosimultaneously capture hazardous particles and chemical gases isdramatically increasing.

Conventionally, synthetic polymers have been employed as the airfiltering material and fabricated into different configuration, such asporous films and non-woven fibrous mats. For non-woven mats, they areusually made of randomly oriented micron-size fibers of plastics, suchas polyethylene and polypropylene. Usually, these traditional air filtermats can only capture particles via four different size-basedmechanisms, including sieving, inertial impaction, interception, anddiffusion. These four mechanisms work together in capturing pollutantparticles according to their sizes. Clearly, these porous fabrics ofconventional plastic micron-size fibers are not effective for removingchemical gases due to their inert surface and/or limited surface area.To remove chemical molecules and odors, other filtration materials, suchas activated carbon (charcoal), have to be used. As a result, in orderto achieve high efficiency for both particulate and chemical pollutions,the air filters have to combine different layers with different removingfunctionalities (such as the combination of activated carbon filter withconventional air filter). This strategy will dramatically increase theair resistance or pressure drop and so, the energy consuming of the airfilter. Therefore, developing a single material with multi-functionalfiltration properties is significant for the development ofcost-effective and high-efficient air-filters.

Soy protein (SP), one of the most abundant and low-cost plant proteins,has been widely studied as a type of biomaterial with differentapplications, including antibacterial, active food packaging, adhesives,tissue engineering, drug delivery, and so on. These significantapplications of SP indicate that SP is a high-performance biomaterialwith multi-functionality. In particular, the ionizable groups, such asglutamic acid, lysine, histidine etc., have been found critical forantibacterial properties. Also, it was reported that the ionizablegroups in soy protein can create active sites to capture bacteria. Sincecharged fibers were reported to be very effective for capturing varioustypes of pollutants, existence of these ionizable groups in soy proteinindicates a great potential to capture charged pollutions. In fact, itis known that, in addition to these ionizable groups, soy proteinpossesses lots of other functional groups, including polar, nonpolar,hydrophobic and hydrophilic ones. These functional groups, such ashydroxyl (—OH), carboxyl (—COOH), amine (—NH₂ and —NH₃ ⁺), methyl (CH₃)etc., make soy protein a very attractive material with ability tointeract with various particles or chemicals. To demonstrate this greatpotential, in this study, soy protein is first denatured and thenfabricated into nanofibers with the help of poly(vinyl alcohol). Thedenaturation combined with the nanofiber morphology can greatly increasethe density of active sites available for interacting with pollutions.The results reveal that SP-based nanofabric can show high removingefficiency in both particulate and chemical pollutions, which, based onthe authors' knowledge, has never been realized by a single materialbefore.

Materials and Methods

Raw Materials and Solution Preparation.

Soy protein isolate powder with >90% protein content was supplied fromADM Foods & Wellness, Decature, Ill. Poly(vinyl alcohol) (PVA; Mn=75,000g/mol) granules were obtained from Sigma-Aldrich St. Louis, Mo. Aceticacid (99.9% purity) was purchased from J.T.Baker® (PA, USA). Two mainprocedures were explored in this study to produce the nanocompositefibrous mats, a powder-based procedure and a solution-based procedure.It was found experimentally that samples prepared via the solution-basedprocedure consistently outperformed the samples prepared via thepowder-based procedure. The powder-procedure sample preparation and theparticulate and chemical filtration (see FIGS. 18 and 19 a, 19 b) dataare described below. Due to the issues that occurred duringimplementation of a powder-based procedure, the solution-based procedurewas chosen for the studies presented in this work. For thesolution-based method, soy protein isolate was thermally denatured inmixed solvent (volume ratio, acetic acid:DI water=80:20) with aconcentration of 8.5 wt % at 85° C. for 4 h using magnetic stirring (400rpm). Poly(vinyl alcohol) was dissolved separately in the same solventwith a concentration of 8.5 wt % at 60° C. for 2 h using magneticstirring (400 rpm). Then the denatured SPI was loaded as a solution intothe PVA solution with different ratios and mixed with PVA solution for24 h using a spin mixer.

Preparation of Soy Protein-Based Filter Nanofabrics.

The soy protein-based nanofibers were fabricated by electrospinningtechnique. The SPI/PVA nanocomposite solution was loaded in a syringe(Monojet™ Kendall) with a 21-gauge blunt tip needle. A voltage of 16-21kV was applied for electrospinning and was controlled by a high voltagepower source (ES50P-5W, Gamma High Voltage Research). A mono-injectsyringe pump (KD Scientific, KDS-100) was utilized to pump the SPI/PVAsolution. Commercial aluminum mesh with wire diameter of 0.011 inch andmesh size of 18 mm×16 mm was grounded to collect the fibers. Thedistance between needle and sample collector was fixed to 10 cm andaverage flow rate of 0.6 ml h⁻¹ was utilized. Moreover, the needleposition (horizontal and vertical) was adjusted continuously duringelectrospinning to achieve uniform fiber mat with controlled nanofiberdiameter and different areal densities.

Pollution Generation and Air Filtering Testing.

Two different source of pollution were utilized to prepare polluted airsamples. The first one was cigarette smoke and the other source ofpollution was the air product of burning plant materials. It has beenestablished that cigarette smoke consists of PM with size from 0.01 to10 μm, and approximately 7000 different chemicals, where hundreds aretoxic such as formaldehyde (HCHO) and carbon monoxide (CO). Samplesproduced from burning plant materials also included broad range of PMparticles and high concentrations of HCHO and CO. The polluted airsamples were diluted in a plastic gas-bag to a hazardous and measurablelevel for the analyzer due to very high initial pollutant concentration.A particle counter (CEM, DT-9881) was used to measure the PM (differentparticle sizes from 0.3-10 μm) and toxic chemicals (HCHO and CO)concentration of the polluted air samples. Also, the air flow resistance(the pressure difference of both sides of air filter) was controlled andmeasured by a monometer (UEi, EM201-B) with a standard air flow velocityof 5 cm s⁻¹. A circular filter sample with diameter of 37 mm was placedin a home-made sample holder to perform air filtration testing for allthe measurements. The air downstream of the filter was collected by aclean vacuum gas-bag. Similar measurements were conducted for thecollected filtered air downstream of the filter. The testing procedurewas performed on four filters fabricated independently (with similarareal density) from the same solution for each type of samples toreplicate the results. Again equation (1) from Example 1 is used todetermine the removal efficiency η.

Characterization.

Scanning electron microscopy (SEM, FEI SEM Quanta 200F) was utilized toinvestigate the change in SPI particle size after denaturation processas well as morphology of the nanofabrics before and after airfiltration. All the samples were sputter-coated with gold nanolayer (10nm in thickness) using (Technics Hummer V) sputter coater. To furtherstudy the denaturation of SPI, transmission electron microscopy (TEM,FEI Tecnai G2 20 Twin) was used to investigate the particle size. Inorder to study the interface interactions between pollutants andnanofabrics, Fourier transform infrared spectroscopy (FTIR, NicoletThermo Scientific) absorption spectra was employed. To distinguish theinteractions between nanofibers and pollutants from the interactionsinside the fabric or polluted-air themselves, the FTIR spectrum ofinclude polluted-air, clean protein nanofabrics before and afterfiltration were compared. All the measurement was repeated for 3 times.

Results and Discussions

Denaturation of Soy Protein.

In order to explore the potential of soy protein isolate (SPI) for airfiltration application, the protein was first denatured to unfold thein-built protein structures. As illustrated in FIG. 10a , pristine soyprotein takes the form of micro-size powders with diameter around 50 μm(see FIG. 10b ). It is well-known that natural proteins usually havefour different levels of structure, including primary, secondary,tertiary and quaternary structures. For simplicity, the raw material,pristine SPI, can be viewed as a big particle hold by numerousintermolecular interactions contributed by 18 types of amino acids inthe protein chains. Therefore, the denaturation process can destroy thein-built high level structures (secondary, tertiary and quaternary) ofthe big SPI particles and finally unfold the soy protein chains asillustrated in FIG. 10c . In order to study the effect of denaturationprocess on SPI morphology, scanning electron microscopy (SEM) andtransmission electron microcopy (TEM) were utilized. The SEM and TEMimages of the denatured SPI particles and after denaturation are shownby FIGS. 10d and 10e , respectively. One can find that the denaturationcan result in a significant reduction in particle sizes from about 50 μmto less than 30 nm. The huge reduction in the particle size indicatesthat more functional groups along the SPI chain will be exposed to thesurface, which is critical for air filtration application. At the sametime, a well-denatured SPI is also important for the electrospinningprocess as well as the quality control of the nanofibers, since it isone of the perquisites for achieving good homogeneity for both theSP/PVA solution and the final nanofibers.

Morphology of SP/PVA Nanofabrics.

As described in the experimental part, cigarette smoke was used as asample of polluted air. Cigarette smoke includes a very complicatedcombination of various particles, toxic chemical molecules and evenheavy metal ions as introduced in Experimental part. This mixture ofvarious particles and hazardous chemical molecules makes cigarette smokea good sample for the evaluation of the filtration performance of theSP/PVA nanofabric. SEM was utilized to investigate the morphologicalcharacteristics of the nanofibrous filter mats with different SPIconcentration and similar areal density (4.50 g m⁻²). FIGS. 11a and 11cand e show the SEM images of the SPI/PVA nanofabrics with differentSPI/PVA ratio before the air filtration testing. From the SEM images, itcan be found that all the SPI/PVA ratios give rise to similar fiberdiameter in the range of 100 to 200 nm (see FIG. 11g and FIGS. 21a-21efor size distribution). It is noted that the pores are irregular innature for the nanofabrics, which makes it difficult to determine thepore geometry. As a result, the pore size was estimated by ImageJ andthe size distribution of these samples was found similar, ca. 4 μm, asshown in FIG. 11h (also see FIGS. 20a-20f ). To characterize thedistribution of the denatured SPI in the SPI/PVA nanofiber, highmagnification of the nanofiber was used to characterize the morphologystructure of a single SPI/PVA nanofiber (see the TEM images in FIGS.21a-21e ). Based on the images, one can find that there is no SPInanoparticles inside the nanofiber or on the nanofiber surface,indicating a good miscibility between denatured SPI and PVA, which isconsistent with other studies. In spite of this fact, it is stillbelieved that there are SPI molecules on the nanofiber surface due tothe following reasons: (1) the air filtration performance for SPI/PVAnanofabrics which will be introduced later is much better than pure PVAnanofabrics, indicating that the surface of PVA nanofiber is verydifferent from PVA/SPI nanofiber; (2) The TEM images (see FIGS. 22a-22c) show that there is no core-shell structure for SPI/PVA nanofiber,indicating SPI is homogeneously distributed in the nanofiber and lots ofSPI molecules exist on the surface since a high loading of SPI was usedfor the nanofiber.

FIGS. 11b, 11d and 11f are the SEM images of the nanofabrics withdifferent SPI/PVA ratio after air filtration testing. The digital photosinserted in FIG. 11c and FIG. 11d display a dramatic color change fromwhite color (clean filter before air filtration) to yellow/orange color(used filter after air filtration), indicating that the SPI/PVAnanofabrics captured lots of pollutions. Although the morphology of theas-spun nanofibers with different SPI concentrations is almost similar,the SEM images after filtration show that the soy protein-based filtermat can capture more particles after exposing to the same polluted airfor the same time (see FIGS. 11b and 11d ). More SEM images of thenanofabrics of SPI/PVA with other ratios can found in FIGS. 23a-23f . Itwas found that the sample with SPI/PVA ratio around 1:1 captured moreparticles than the rest of the samples, which indicates an optimalloading of SPI for good air filtration performance. The existence of anoptimal SPI loading is mainly due to the fact that the air filtrationperformance is determined by two factors related to SPI loading at thesame time: the mechanical strength and surface activity of thenanofabrics, which will be further discussed in the air filtrationperformance studies later.

Air Filtration Performance

In order to test the filtering capabilities of the protein-basednanofiber mats, we performed both efficiency test (η %) and the pressuredrop test (ΔP). For a standard high efficiency filter, the PM_(2.5)efficiency is suggested to be 95-100%, and with regards to a HEPAfilter, the requirements state that it must fulfill a removal efficiencyof 99.97% for the most penetrating particle size (MPPS) of 0.3 μm with amaximum pressure drop of 1.3 in H₂O gauge (˜325 Pa) at an air facevelocity of 5 cm s⁻¹, as suggested by the US Department of Energy (DOE).

Removal of Particulate Pollutants.

For this study, we first focused on the effect of SPI concentration onthe morphology of the nanofabrics as previously shown in FIGS. 11a-11h .The filtration efficiency for both PM_(2.5) and PM_(10-2.5) of thesesamples was then tested. In order to compare the performance, all thesesamples were prepared with similar areal density (ca. 4.5 g m⁻²). Asshown in FIG. 12, the removal efficiency for small particles (PM_(2.5),that is, particles with size less than 2.5 μm) and big particles(PM_(10-2.5)) are compared for SPI/PVA nanofabrics with differentSPI/PVA ratios. Based on FIG. 12, it can be found that the removalefficiency of PM_(10-2.5) (particles with sizes between 2.5 and 10 μm)stays in the same range of around 99.90-99.99%, regardless of the SPIloading. This result indicates that large particles are mostly capturedthrough sized-based physical mechanisms of filtration. However, forPM_(2.5), the removal efficiency as displayed in FIG. 12 is dependent onthe SPI/PVA ratio and lies in the range of 99.40-99.80%. In particular,the neat PVA nanofabric mats show a PM_(2.5) removal efficiency of99.45%, while the SPI/PVA nanofabrics with 1:1 ratio leads to a higherefficiency of 99.80% for PM_(2.5). It is noted that an improvement by0.4% is significant since the efficiency is approaching the limit, 100%.The above result reveals that contribution of SPI is critical forimproving the removal efficiency for small particles with size less thanthe pore size (ca. 4 μm). This result also indicates that the smallparticles were removed via the interaction-based mechanism with thecritical contribution from the multiple functional groups in the SPIstructure, instead of the sized-based mechanisms. This conjecture willbe further confirmed by the filtration performance of toxic chemicals aswill be discussed later.

From FIG. 12, it can also be found that the sample with PVA/SPI ratioaround 1:1 gives rise to the best air-filtration performance in terms ofremoving PM_(2.5). This result can be explained as follow. Thefiltration performance of the composite nanofabric is dependent on notonly the PVA/SPI ratio, but also the mechanical strength of thenanofabrics. In our study, it was found that, when the SPI/PVA is higherthan 1:1, the PVA/SPI nanofiber becomes brittle, which leads toformation of micro-cracks or large pores in the nanofabrics after theelectrospinning or during the filtration testing (see FIGS. 24a and 24b), which is consistent with other studies. As a result, there should bean optimal loading for SPI to achieve a good balance between mechanicalstrength and surface activity. In brief, the structure and mechanicalweakness of PVA/SPI nanofiber with higher SPI/PVA ratio (higher than1:1) are the main reasons for the efficiency drop. Therefore, in thefollowing studies, all the samples will use a PVA/SPI ratio of 1:1.

In addition to the SPI/PVA ratio, another critical parameter affectingthe air filtration performance is the area density of the nanofabrics.In FIG. 13a , the removal efficiency for pollutant particles withdifferent sizes are compared among the nanofabric samples with differentareal densities. From these results, it can be observed that, forparticles with size larger than ca. 1 μm, the removal efficiency doesnot change significantly with the increasing of areal density. However,for particles with size less than 1 μm, the removal efficiency is highlydependent on the areal density until it reaches ca. 4.50 g m⁻², wherethe removal efficiency has reached its maximum point. With regards tovery small particles of PM₀₃, an improvement in removal efficiency from86.40% to 98.70% is achieved when the areal density increases from 1.55g m⁻² to 4.50 g m⁻². However, no significant increase in removalefficiency for the PM_(0.3) can be found when the area density is abovethe 4.50 g m⁻². These results show that increasing the areal density ofthe nanofabrics can remarkably improve the removal efficiency of smallparticles but not larger particles. This phenomenon can be explained asfollowing. Firstly, it is probably determined by an interaction-basedmechanism for particles that have smaller size than the pore size of thenanofabrics. Secondly, a higher areal density of nanofabrics willincrease the contact possibility between small particles and thenanofabrics, as well as the chance to capture more small particles. FIG.13b further displays how the areal density affects the removalefficiency for PM_(2.5) and PM_(10-2.5). From this figure, it can befound that the area density doesn't affect much the removal efficiencyfor PM_(10-2.5), but affect significantly on that for PM_(2.5), which issimilar to the situation for PM_(0.3) as introduced previously. It isalso found that, above areal density of 4.50 g m⁻², there is nosignificant improvement in the removal efficiency of PM_(2.5). However,below 4.50 g m², PM_(2.5) removal efficiency increases with theincreasing of areal density. Therefore, it can be concluded that anappropriate areal density of protein nanofabrics is important to achievehigh removal efficiency for small particles. The high removal efficiencyfor small particles is most likely due to the “nano-size” effects and“active” surface properties of the protein-based nanofiber, which canhelp to trap very small particles with size below the pore size of thenanofabrics.

Removal of Toxic Chemicals.

In addition to a high efficiency for removing PM with different sizes,the protein-based nanofabrics also show excellent removal efficiency fortoxic chemicals. In this study, we chose formaldehyde (HCHO) and carbonmonoxide (CO) molecules to test the chemical removal performance of theprotein-based nanofabrics. HCHO and CO are the cancer-causing andpoisonous gases that exist in cigarette smoke. FIG. 14a summarizes thechemical removal efficiencies of HCHO and CO for the samples withdifferent SPI/PVA ratios. For the HCHO removal efficiency, this figuredemonstrates that the overall range of HCHO removal efficiency isbetween 30.0% and 62.50%. In particular, the neat PVA nanofabric shows amuch lower HCHO removal efficiency (ca. 31.23%) than SPI/PVA nanofabrics(ca. 62.50% for the sample with SPI/PVA ratio of 1:1), indicating thatthe SPI plays a critical role in removing chemical gases. For CO removalefficiency, all the nanofabrics, including pure PVA, show goodfiltration performance and higher than that of removing HCHO, i.e. theremoval efficiency for SPI/PVA samples ranges from 76.90% to 90.90% and,for pure PVA sample, it is 55.67%, which is much lower than that ofSPI/PVA samples. Similar to the particulate removal efficiency, there isalso an optimal ratio for PVA/SPI which gives rise to the best airfiltration performance in terms of removing particulates and toxicchemicals. As a comparison, the removal efficiency for HCHO and CO bycommercial HEPA filters which have no functional groups along theirfibers was also tested. As shown in FIG. 14a , the removal efficiencyfor HCHO and CO are less than 5% and 3%, respectively, even though theyhave a much higher areal density of 164 g m⁻². Since toxic gases aremolecules with sizes much smaller than particles in the polluted air, itis believed that the removal of toxic chemicals is governed by aninteraction-based filtration mechanism that is contributed by soyprotein structure, which will be analyzed later.

The effects of areal density on the chemical removal efficiency for HCHOand CO are shown in FIG. 14b . The results show that the removalefficiency for toxic gases doesn't change significantly with theincreasing of areal density of the nanofabrics. For removal efficiencyof HCHO, it fluctuates between 42.50% and 62.50% and, between 78.90% and85.70% for CO. This result may suggest that the removing of toxic gasesis a very slow process and the change of the area density doesn't leadto a big difference in the time for the absorbing of toxic gases. Fromthe above results, one can conclude that the combination of nanofibermats with the functional SPI on the surface provides a promisingsolution for multi-functional air-filtering materials.

Pressure Drop and Quality Factor.

Beside particulate and chemical removal efficiency, pressure drop or airflow resistance of an air filter is another parameter related to thefiltration performance. As suggested by US DOE, the pressure drop shouldbe less than ca. 325 Pa at an air face velocity of 5 cm s⁻¹. FIG. 15ashows a quantitative study on how the air flow resistance change withthe areal density of the sample with SPI/PVA ratio of 1:1. It is shownthat the pressure drop generally increases nonlinearly with the arealdensity, which may be related to a complicated change in the porousstructures of the nanofabrics when more and more layers of nanofibersare added. Taking into account the particulate removal efficiency (seeFIG. 12) and the pressure drop (see FIG. 15a ), an optimized arealdensity has been experimentally determined to be around 4.50 g m⁻². Itis noted that the optimal areal density should also be related to thesize of the nanofibers as well as the porous structures of thenanofabrics, which is beyond the scope of this study. In order tocorrelate the pressure drop and removal efficiency for evaluating theoverall performance of the SPI-PVA nanofabrics, quality factor (QF),that is, the figure of merit (FOM), has been calculated for thefiltering materials studied in this work. QF is representative of theratio between the particulate removal efficiency of the air filter andthe pressure drop due to air flow across the filter. The QF parameterrepresents a comprehensive evaluation of the air filtering performance.For a good air filter, it should give rise to a high QF number, whichmeans the air filter can achieve high removal efficiency with lowpressure drop or air resistance. A QF comparison between different typesof air filtering materials and the SPI/PVA nanofabrics is shown in FIG.15b . Two types of commercial air filters along with the reported,PAN-85 air filter, are shown in this figure for comparison. Itdemonstrates that the optimized PVA/SPI nanofabric possesses the highestQF (ca. 0.027) amongst these different types of air filtering materials,which indicates that the protein-based nanofabric possesses the bestfiltration performance.

The time-dependent behavior of the filtration performance for theSPI/PVA nanofabrics was also studied. Time-dependent behavior is relatedto the long-term performance of an air filter material. For theprotein-based nanofabrics, the time-dependent air filtration performancewas studied via investigating how the removal efficiency and weight-gainof pollutions depend on the using time. For simplicity, the optimizedSPI/PVA nanofabric was employed to compare with other counterparts. Thesamples were exposed to a highly polluted air from cigarette smoke forabout 180 min. After each time interval of 45 minutes, the particulateand chemical removing efficiency, the weight-gain of pollutions wererecorded. (1) Time-dependent behavior for particulate removalefficiency. As shown in FIG. 16a , the particulate removal efficiencyfor both SPI/PVA nanofabrics and commercial HEPA decreases with testingtime. It can be found that the PM_(2.5) removal efficiency for SPI/PVAdecreases slightly faster than that for commercial HEPA. This is becausethat the commercial HEPA has a much higher areal density (ca. 160 g/m²)than our SPI/PVA nanofabric (4.5 g/m²). (2) FIG. 16b displays the toxicchemical removal efficiency vs. testing time. For the protein nanofabricsample, the HCHO removal efficiency drops steadily from 63.28% to 34.78%after 45 minutes of testing time. While the CO removal efficiency dropsfrom a high value of 85.78% to 62.53% after 45 minutes, and further to33.33% after 90 minutes of testing. In comparison, for the commercialHEPA filter, the chemical removing efficiencies for both HCHO and CO arebelow 5% and decrease to be less than 3% after about 180 min. oftesting. This decreasing behavior of the removal efficiency is mainlydue to the fact that less and less active sites on the nanofiber surfacewill be available for pollutions absorption as the filtration going.

To further demonstrate the advantages of the SPI/PVA nanofabrics inremoving pollutions, the weight-gain of captured pollutions was recordedafter each time interval. It is shown that the total weight of thecaptured pollutions increases from 3.5 mg to 7.3 mg after 180 minutes oftesting. However, the commercial HEPA filter shows only a slight weightincrease from 2 mg to 3.8 mg after the same testing time. To betterdemonstrate this advantage of the protein-based nanofabrics, a ratiodefined as W_(p)/W_(f) (W_(p), the weight-gain of captured pollutants,W_(f), the weight of the filter before testing) is employed here todescribe the ability to capture pollutions. It can be found from FIG.16c that the ratio W_(p)/W_(f) for PVA/SPI nanofabric increases from0.72 to 1.5 when the testing time increases from 5 min. to 180 min. Incontrast, the W_(p)/W_(f) ratio for HEPA filter increases from only 0.01to 0.02 for the same time interval. A high W_(p)/W_(f) ratio indicates astrong ability to capture pollutants. The value of 1.5 for W_(p)/W_(f)reveals that the SPI/PVA nanofiber can capture an amount of pollutionswith a weight even much higher than the weight of the filter materialitself. This behavior is similar to spider web which has really lowweight but can capture huge particles. The above results indicate thatthe protein-based nanofabrics can simultaneously improve the removalefficiency for both particulate and toxic gases and long-termperformance due to an enhanced capturing mechanism for both particlesand toxic gases.

Filtration Mechanism.

To further understand the unique performance of the SPI/PVA nanofabrics,an interaction-based capturing mechanism is proposed based on examiningthe chemical characteristics of both the cigarette smoke pollutants andthe nanofabrics before and after filtration test. Conventionally, theremoval efficiency for particles is mainly dependent on the morphologyof the filter mats due to the four primary sized-based filtrationmechanisms. Nanofibers compared with micron-size fibers possess largersurface area and higher surface energy which can dramatically improvethe interaction with the PM particles and enhance the efficiency.Moreover, as it is mentioned before, SPI possess numerous functionalgroups which can interact with different types of particles and toxicchemicals in the polluted air. The strong interactions between PVA/SPInanofabrics and pollutions (see FIG. 17a ) will enhance the capturingcapabilities for both toxic chemicals and solid particles. A simplifiedschematic illustration of the possible interactions among PVA, SPImolecule, PM particle, and formaldehyde (as examples of differentpollutants in the air) is shown in FIG. 17a . FIG. 17b is the SEM imageshowing SPI/PVA nanofabrics strongly interacting with pollutions afterfiltration test. It can be found that the aldehyde groups existing informaldehyde can interact with both carboxylic and amine groups of thesoy protein. This interaction can result in formation of aldimine bondswhich is the main reason for the color change of the filter to yellowishafter the filtration test (see the insert photos in FIGS. 11c-11d ).Moreover, solid particles and other toxic chemicals with differentcompositions can undergo various types of interactions includingcharge-charge interaction, polar-polar, hydrogen bonding and etc. Thisschematic demonstrates an enhanced mechanism based on strongfiber-pollutants interactions.

To further characterize the interactions between pollutants andprotein-based nanofibers, Fourier transform infrared spectroscopy (FTIR)was employed to investigate the functional groups existing in thepolluted-air, clean SPI/PVA nanofabrics, and nanofibers with capturedpollutants. The FTIR spectra of the cigarette smoke is shown in FIG. 17c. The main peaks are around 3,649, 2,360, 1,653, 1,558, 1,506, and 1,456cm⁻¹, which indicate the existence of O—H, C—H:H—C═O (formaldehyde),C═O, and C—O (last three peaks) groups, respectively. These groups inpolluted-air can strongly interact with the functional groups existingon the surface of nanofibers. By comparison of the FTIR spectra of neatPVA and SPI/PVA nanofibers (see FIG. 17d ), one can easily identify theSPI in the nanofibers via amide groups. Although there is no new peakgenerated between SPI/PVA nanofabrics and pollutions, a significantchange in the intensity of specific groups/interactions (e.g. O—H, COOH,N—H and C—N functional groups) was observed and the change of peakintensity is summarized in FIG. 17e . The reason why no new peak wasgenerated after the filtration testing is possibly that the interactionsbetween pollutants and the SPI/PVA nanofabric are covered by thoseinteractions existing in the protein. As a result, one can only observean increase in the peak intensity, instead of new peaks, afterfiltration.

Conclusions

In summary, this study demonstrates a high-performance multi-functionalair filtration nanofabric materials produced from protein/polymercomposites. The combination of abundant plant protein with porousnanofabrics provides a promising solution to “green” and high-efficientnanomaterials for air filtration applications. The protein-basednanofabric shows high removal efficiency for both types of pollutants:particles with a broad size range and toxic gases with variouscharacteristics, which has never been reported from a single air-fibermaterial. The soy protein is employed as an example to developsustainable and environmental friendly nanomaterial for air filteringapplications. This study indicates that the amino acids of proteins cansignificantly enhance the interactions between nanofabrics andpollutions, which is especially critical to capture the particles withsize much smaller than the that of the pores and gases molecules.Moreover, the protein-based nanofabrics are able to improve the removalefficiency of air pollutions while decrease the air flow resistance,both of which are the most crucial factors for practical applications.In short, this study indicates that protein-based nanofabric is apromising green nanomaterial with great potential to deal withcomplicated pollutions in the air due to an enhanced-interactionmechanism.

Additional Information

Additional data and figures for the filter pore size and distribution,digital images of protein-based nanofabrics, filtration performance ofthe samples prepared by powder-based procedure, SEM images before andafter filtration test, particulate and chemical removal efficiencies offilters with different protein loadings are disclosed below and in FIGS.18-24.

Powder-Based Method for Sample Preparation:

Regarding the powder-based procedure, PVA powder was dissolved in 80%(v/v) aqueous acetic acid at 60° C. for 2 hr under magnetic stirringconditions (400 rpm). After the PVA had been fully dissolved in thesolvent, SPI powder was added in various loadings to the PVA solutionand subjected to the same magnetic stirring conditions at 85° C. for 24hr in order to denature SPI in presence of the dissolved PVA. Thepowder-based procedure samples displayed significantly decreasedelectrospinability. There was also an issue of the amount of SPI loadingpermissible in the powder-based samples where above a 1:1 for SPI to PVAratio, the powder-based procedure failed to produce samples that wouldsurvive during the performance testing due to the brittleness of thefiber mat. Considering the disadvantages of power-based method, wefinally choose a solution-based or denatured-based method as describedin the experimental part of the manuscript.

Example 3: Hybrid of Natural Protein Nanofibers with Cellulose Fibersfor High Performance Air Filtration Application 1. Introduction

Air pollution has become a major environmental concern due to the hugeamount of pollutants produced from vast human activities. It containsnumerous combinations of pollutants such as particle matter (PM) ofvarious sizes, chemical mixtures, biological hazards, and etc. Moreover,creation of unexpected chemical compounds due to the photochemicalreactions in the polluted air, makes it more and more puzzling to cleanthe air. These complicated mixtures have posed excessive threats topublic health. PM contains small solid particles and liquid dropletswith different sizes. Regarding the size, particulate pollutants can becategorized by PM_(2.5) and PM_(10-2.5), indicating particle sizes below2.5 and between 2.5 and 10 respectively. PM_(2.5) is mainly one of themajor pollutants in many developing countries. These particles arecommonly composed of organic (e.g. carbon derivatives species such ascarbon oxides) and inorganic (e.g. nitrates, sulfates, silicates, etc.)compounds which can seriously influence the air quality, public health,climate change, air visibility and so on. In addition, polluted airincludes numerous types of toxic gaseous molecules, such as sulfuroxides (SO_(x)), nitrogen oxides (NO_(x)), carbon oxides (CO and CO₂),formaldehyde (HCHO), methane (CH₄), and a mixture of other volatileorganic compounds (VOCs). These chemicals can undergo variousphotochemical reactions which may lead to the creation of unexpectedhazardous pollutants. Biological hazards including bacteria, viruses,mites, pollen and etc. can trigger many allergic reactions andinfectious illnesses such as influenza, measles and chicken pox. Becauseof the intensive effects of these pollutants on the environment andhuman health, providing an effective protection, particularly towardimproving the indoor air quality, is urgently needed.

Filtration membranes are commonly used to remove the pollutants from theair and improve the quality of the air. Some attempts have been made forenhancing the outdoor personal protection, and improving the indoor airquality. An ideal air filter should have a high removal efficiency ofpollutants yet maintaining low resistance to the air flow. Conventionalair filters are usually made of micron-size fibers of synthetic plasticssuch as polyethylene and polypropylene. These air filters areineffective for removing the toxic gaseous chemicals from the air due tothe lack of active functional groups in the structure of the rawmaterials. These materials are only effective for capturing particulatepollutants based on the four primary physical and size-based filtrationmechanisms, including sieving, interception, impaction, and diffusion.In our previous work, we have found that the natural protein-basednanofabrics can provide multifunctional air filtration capabilities withvery high affinity to various pollutants. These protein-basednanofabrics showed extremely high removal efficiencies for both solidparticles with different sizes and various toxic gaseous chemicals whilemaintaining a very low resistance to air. These capabilities make itpossible to use thin layers of the protein-based nanofabrics on asubstrate to develop high efficiency air filtering materials forpractical filtration applications.

Cellulose is the most abundant polymer in nature with low price and highbiodegradability. It can be derived from a variety of sources, such aswoods, annual plants, microbes, and so on. Cellulose is a linearpolysaccharide made of β(1→4) D-glucose units that contains manyfunctional groups such as methylol, hydroxyl, and etc. The elementaryfibrous structure of cellulose leads to the specific strength and highperformance properties, including high mechanical strength as well asflexibility. Cellulose has been studied extensively as wastewatertreatment filtration membranes, films, hydrogels and aerogels, andenergy harvesting. In addition, natural proteins, such as gelatin andsoy protein, are ones of the most abundant biopolymers. It is well-knownthat proteins are rich in functional groups including numerous aminoacids in their chemical structure. They can strongly interact withvarious pollutants, both solid particles and toxic gaseous chemicals,via numerous types of interactions. Therefore, the functional groupsmake proteins an ideal material for air filtering applications.

In this work, our goal is to achieve high efficiency and multifunctional“green” air filters. To this end, we prepared a hybrid structure that ismade of a thin layer of protein-based nanofibers (either gelatin or soyprotein-based material) and porous cellulose fiber mat as the substratelayer. Paper towels are cost effective porous materials made ofcellulose fibers. We assumed that cellulose-based paper towels cancontribute to the air filtration performance due to the porous networkin their fibrous structures. We also hypothesized that paper towelcannot only capture the particulate pollutants via primary physicalfiltration mechanisms, but also the active functional groups in thestructure of cellulose may interact with pollutants including toxicgaseous chemicals in the air. The protein-based nanofiber layer (eitherpure gelatin (G) or soy protein-based composite (SC)) was deposited onthe paper towel substrate. It is noted that the nanofibers have manyadvantages over the micrometer fibers, such as extremely high surfacearea and surface energy. These characteristics may significantlyincrease capability of capturing more pollutant substances from the air.The morphology and filtration performance of the hybrid air filteringmaterials as well as their component materials were studied. Finally,the mechanisms the hybrid air filtering materials for their highfiltration efficiency of simultaneously capturing particulate pollutantsand toxic gaseous chemicals were analyzed.

2. Materials and Methods

2.1. Raw Materials and Gelatin Solution Preparation

Three types of cellulose-based paper towels (Scott® PT-T (textured),Scott® PT-P (plain), and Bounty® PT) with different surfacemorphology/texture were provided. Gelatin powder (type A, from porcineskin, Sigma Aldrich) was purchased. Soy protein isolate (SPI) powderwith >90% protein content was supplied from ADM Foods & Wellness,Decatur, Ill. Granules of poly(vinyl alcohol) (PVA; Mn=75000 g/mol) wereobtained from Sigma-Aldrich, St. Louis, Mo. Glacial acetic acid (AcOH,purity=99.9%) was purchased from J.T.Baker® (PA, USA). Gelatin solutionwas prepared in a mixture solvent (AcOH:DI water=80:20, volume ratio)following out previous study to achieve a homogenous yellow solution forelectro spinning.

2.2. Preparation of Protein Nanofiber-Coated Paper Towel Filter Mats

Gelatin nanofibers were fabricated via facile electrospinning technique.A mono-inject syringe pump (KDS-100, KD Scientific), a plastic syringe(Monojet™ Kendall) and a 21-gauge blunt tip needle were used to pump ofthe gelatin solution. An operating voltage of 19-24 kV was applied andcontrolled using a high voltage power source (ES50P-5W, Gamma HighVoltage Research) to draw the nanofibers. Paper towel substrate wasfixed on a grounded commercial aluminum mesh with wire diameter of 0.011inch and mesh pore size of 1 mm×1 mm to collect the nanofibers and coatthe paper towel surface. The needle-collector distance was fixed to 15cm and a controlled feed rate of 0.5 ml/h was utilized. Duringelectrospinning, the horizontal and vertical position of the needle wascontinuously regulated to deposit a uniform nanofiber mat withcontrolled diameter and thickness on the paper towel substrate. TheSPI/PVA nanofabrics were fabricated following out previous work.

2.3. Polluted Air Sample Generation and Air Filtration Measurements

Tobacco smoke was selected to prepare the polluted air sample fortesting. It is well known that tobacco smoke is rich in various PMparticles ranging from 10 nm to more than 10 μm, numerous toxic gasesand carcinogens (e.g. formaldehyde (HCHO), sulfur dioxide (SO₂), carbonmonoxide (CO), and many other volatile organic compounds (VOCs)), andseveral heavy metal ions. This combination of pollutants makes thetobacco smoke an appropriate source of pollution for air filtrationperformance testing. A plastic vacuum air-bag was utilized to collectthe tobacco smoke. Due to the extremely high initial concentration ofpollutants, the polluted air sample was diluted to a hazardous levelwithin the analyzer measurable range. PM particles with different sizes(0.3-10 μm) and two toxic chemicals (HCHO and CO) concentration wasdetected using a particle counter (CEM, DT-9881). Moreover, a portablegas detector (GMI PS500) was utilized to measure the concentrations ofSO₂ and VOCs of the polluted air sample. Also, the pressure drop(pressure difference between upstream and downstream of the filter) wasmeasured by a differential pressure gauge (EM201-B, UEi) at differentair face velocities. In addition, a portable air sampler (MiniVol AirMetrics, Eugene, Oreg., USA) was utilized to test the air filtrationproperties at different air flow rates. A circular filter sample withdiameter of 37 mm was placed in a home-made sample holder to perform airfiltration testing for all the measurements. The filtered air was thencollected in a clean plastic vacuum air-bag and similar measurementswere carried out for the filtered air sample. To replicate the results,the testing process was executed on four composite filter mats preparedindependently. One can calculate the removal efficiency (E %) using thefollowing equation (5).

$\begin{matrix}{{E(\%)} = {\frac{\left( {C_{u} - C_{d}} \right)}{C_{u}} \times 100}} & (5)\end{matrix}$

where C_(u) and C_(d) are the pollutant concentrations in the pollutedair sample and in the filtered air sample, respectively.

2.4. Characterizations

Scanning electron microscopy (SEM, FEI SEM Quanta 200F) was used tostudy the morphology of the different paper towels and gelatinnanofiber-coated PT filter mats. All samples were coated with platinumnanolayer (3 nm in thickness) using Cressington high resolution sputtercoater. In order to study the interaction-based filtration mechanisms,Fourier transform infrared spectroscopy (FTIR) transmittance spectra wasengaged. The FTIR spectrum of pure SO₂ gas sample before and afterfiltration was utilized to study the toxic chemical filtrationmechanism. All the measurement was repeated for 4 times.

3. Results and Discussion

3.1. Filtration Analysis of Paper Towels and Preparation of the HybridFilters

In order to study the air filtration performance of the hybrid airfilters (protein-based nanofiber coated on paper towel substrate),first, the potential of paper towels (PTs), as a bio-based substrate isinvestigated. To this end, three types of paper towels, i.e. Scott® PT-T(textured surface), Scott® PT-P (plain surface), and Bounty® PT, areselected. Their characteristics, including surface morphology/texturefiber areal density, fiber diameter and thickness as well as their porestructures, are analyzed. The results are shown in FIGS. 25a-25f and inTable 1. The detailed morphology of the selected paper towels wasobserved using scanning electron microscopy (SEM). From the SEM imagesof the fibrous structure of different PT mats shown in FIG. 25a-25c , itcan be found that all the PTs have the cellulose fiber diameters withinthe range of 10 to 13 μm. The sizes and size distributions of the fibersfor the different PTs are almost similar, their pore sizes and pore sizedistributions are very different. As it is shown in Table 1, the BountyPT possesses the smallest pore size of 22 μm. Scott® PT-T has largerpore size of 58 μm and highly textured surface than that of the Scott®PT-P with an average pore size 52 μm. The digital images clearlyindicate the differences in the surface texture of the paper towels. Inaddition, the pore size and distribution of the PTs are characterizedvia ImageJ software, although the irregular pore morphology made it verychallenging to characterize the accurate pore geometry. Among thesethree paper towel products, Bounty® one possesses the highest arealdensity, thickness and average fiber diameter but its average pore sizeis the lowest. Both Scott® paper towels show similar numbers on thosecharacteristics except the thickness values. As paper towels areflexible cellulose-based fibrous mats with high porosity, it is believedthat the pore structures of the fiber mats can significantly impact theair filtration properties, in particular it will affect the air flowresistance, or pressure drop of the filter. This is because (1)different pore structure can result in different air flow path waysthrough the filter, which can affect the pressure drop of the filtermat; (2) the particulate air filtration performance can be highlyinfluenced by the porous structure due to the physical and size-basedcapturing mechanisms of filtration. Therefore, it is necessary toexamine how the pore structures of the paper towels along with theirsurface textures impact their filtration performance, in order to selectthe appropriate paper towel as the substrate for making the hybrid airfilters with the optimized air filtration performance via combining itwith the protein nanofiber coatings. In particular, we need to knowwhich one of the two Scott® paper towel products that have similar porestructures (average size and size distribution) is better for achievinghigher performance from the resulting hybrid filter.

The filtration properties including particulate filtration efficiency(E_(p) %) and the air flow resistance (known as pressure drop, ΔP) ofthe three selected paper towels were tested. Pressure drop or the airflow resistance is one of the critical parameters related to the airfiltration performance. Thus, we first focused on the pressure drop ofthe different paper towels, which were tested using standard 4 lit/minair flow rate. According to the US Department of Energy (D.O.E) theaccepted pressure drop for a high efficiency particulate air filter isless than 325 Pa at a standard air flow rate. Our testing results areshown in FIG. 25d . It can be seen that the difference between the twodifferent Scott® PTs is small as they have similar fiber morphology andpore structure. In specific, the pressure drop of the Scott PT-T (168Pa) is slightly lower than that of the Scott PT-P (173 Pa) because ofits a little larger pore size, while Bounty® PT has the highest pressuredrop (196 Pa), which is not favorable for our selection for the hybridfilter fabrication. The removal efficiency for particulate pollutantswith different particle sizes, PM_(0.3) (0.3 μm), PM_(2.5) (smaller than2.5 μm) and PM_(10-2.5) (2.5-10 μm), are compared among the differentpaper towels. From the results shown in FIGS. 25e and 25f , it can beobserved that all the three paper towel products have very highPM_(10-2.5) removal efficiency. In addition, Bounty® PT possesses thehighest PM_(0.3) removal efficiency (44.42%) and PM_(2.5) (55.60%),compared with that of the two Scott® PTs, both of which possess theremoval efficiency of ca. 9% for both PM_(0.3) and PM_(2.5). It is knownthat the particulate filtration performance of a filter is governed byfour physical and size-based mechanisms. Bounty® PT has the smallestpore size and highest areal density. This means the Bounty PT includemultiple fiber layers resulting larger thickness than the Scott® papertowels because all of them have similar fiber diameter. This is whyBounty® PT possesses higher particulate removal efficiency than Scott®PT.

It is noted that although the Bounty® PT possesses the highestparticulate removal efficiency for all the particle sizes, it also showsthe highest pressure drop value. Therefore, by considering the pressuredrop values and filtration efficiency of the three paper towels, ScottPT-T was selected as the substrate (low pressure drop and high removalefficiency for big particles, i.e. good PM_(10-2.5) performance) forfurther studies.

TABLE 1 Physical properties of different paper towels Samples PropertiesScott ® PT-T Scott ® PT-P Bounty ® PT Thickness (μm) 108 81 183 FiberAreal Density 53 50 68 (g/m²) Avg. Fiber Diameter 10.4 11.67 12.79 (μm)Avg. Pore Size (μm) 58 52 22 Surface Texture Textured Plain HighlyTextured

As paper towels are only effective for removal of particles with sizeslarger than ca. 2.5 μm, but ineffective to capture small particles fromthe air, we combined paper towel with the nanofibers made of proteinsthat have been proved to possess high filtration performance for smallparticles and chemicals previously. In specific, a thin layer of proteinnanofibers, gelatin (G) or soy protein-based composite (SC), was coatedon the paper towel substrate. As it is shown in FIG. 26, thecellulosed-based paper towel made of micron size cellulose fibers canprovide mechanical support and filtration capabilities for largerparticles as indicated above. The protein nanofibers were fabricated onthe cellulose-based paper towel substrate via facile electrospinningtechnique. The nanoscale protein fibers have a very high surface areaand surface energy for removal of more pollutants from the air. Moreimportantly, the functional groups existing in the structure of thecellulose and natural proteins can enhance the filtration capabilitiesfor more types of pollutants. The digital photos shown in FIG. 26indicate that the nanofiber layer can be peeled off and the proteinnanofiber-coated PT filters are foldable. To study the effects of theprotein nanofibers on the filtration properties, different filtersamples that are made of protein based nanofibers, including soyprotein-based and gelatin nanofibers were fabricated on the Scott® PT-Tsubstrate. It is known that soy protein material is brittle. In order toeasily make the soy protein nanofibers via electrospinning method,poly(vinyl alcohol) (PVA) was mixed with denatured soy protein isolate(SPI) to form the soy protein composite for making the nanofibers. Thedetailed information of all the filter samples are summarized in Table2.

TABLE 2 Detailed properties of the single layer and Hybrid filter matsamples Areal Avg. fiber Avg. pore Sample Thickness density diametersize name Material (μm) (g/m²) (nm) (μm) Single PT Cellulose Papertowel: 108 53 1.04 × 10⁴ 58 material fibers (Scott ® PT-T) filters GProtein Pure gelatin 11.33 2.25 70 4.4 nanofibers SC Soy protein^(a)Soy-comp 11.87 2.28 136 4.2 composite nanofibers Hybrid (informationon PT for below samples Nanofiber layer material is shown in above PT)Filters ^(b)SC/PT Soy protein- SPI comp nanofibers/ 12.34 2.43 143 4.3based Paper towel ^(b)G/PT Gelatin- Gelatin nanofibers/ 10.85 2.24 874.1 based Paper towel ^(b)PT/G Paper towel/Gelatin 11.12 2.18 91 4.3nanofibers ^(b)G/PT/G Gelatin nanofibers/ 2 × 6 2 × 1.1 84 4.4/ /74.1Paper towel/ Gelatin nanofibers ^(a)Soy protein isolate (SPI) andpoly(vinyl alcohol) (PVA) with 1:1 ratio. ^(b)Air flow side.

3.2. Hybrid Filters: Gelatin Nanofiber-Coated Paper Towel

3.2.1. Morphology of Gelatin Nanofiber-Coated Paper Towel Filters

The morphological characteristics of pure gelatin nanofibers and gelatinnanofiber-coated paper towel were studied first. The areal density forthe pure gelatin nanofiber sample and the gelatin nanofiber layer in thehybrid materials is similar: ca. 2 g/m². FIGS. 27a and c show the SEMimages of back surface and front surface of pure gelatin nanofibersbefore filtration testing, respectively. It can be seen that theultrafine gelatin nanofibers (70 nm) possess uniform fiber distributionon both sides. Additionally, FIGS. 27e and 27g present the back sandfront surface of the gelatin/paper towel (named as G/PT) before the airfiltration testing, respectively. From the SEM images, it can beobserved that gelatin nanofibers with the average fiber diameter of 87nm are uniformly coated on the micrometer size PT fibers (11 μm). It isknown that a porous structure is one of the important parametersaffecting the air filtration performance of the filter. The pore size ofthe PT samples, pure gelatin nanofiber mats and the hybrid sample G/PTwere summarized in Table 2. FIGS. 27b and 27d show the morphology of theback and front surface of the pure gelatin nanofibers after filtrationtesting, respectively. From the SEM images for the front side of thenanofibers it can be seen that lots of pollutants were captured as thepolluted air passes through the filter; however, the back surface of thenanofibers remains clean. This result can be explained as that thepolluted air encounters with nanofibers once and the front nanofiberlayer remove the pollutants from the air, thus causes the innernanofibers to have less chance to encounter with the pollutants, i.e.less opportunity of removal action of the pollutants from the air. FIGS.27f and 27h show the back and front surface of the hybrid G/PT filtersample after being subjected to the air filtration testing,respectively. The SEM images illustrate that the G/PT fibers were ableto capture huge number of particles after exposing to the same pollutedair for the same period of time. More significantly, it is found thatthe back surface of the nanofibers that were coated on the paper towelsubstrate also contributed to the removal of pollutants. It is believethat the paper towel substrate causes a circulatory air flow between theprotein nanofiber layer and the micron-sized cellulose fibers thatincrease the chances of the pollutants to encounter with nanofibers, andthen the pollutants were captured. This can be confirmed by thefiltration results shown in FIGS. 28a-28d . This phenomenon issignificant and can promote the air filtration performance. The digitalphotos inserted in FIGS. 27g and 27h show obvious color change from amilky color for fresh air filter (before filtration) to a yellow/browncolor (after filtration). This color change indicates that thenanofibers captured various pollutants from the air including both solidparticles and gaseous chemicals.

3.2.2 Air Filtration Performance

Filtration of Particulate Pollutants. In this section, we firstinvestigated the effects of nanofiber configuration on the particulatefiltration properties of the composite filter mats. In order to studythe effect of nanofiber configuration on the filtration performance,three types of samples were prepared. One is that, the gelatinnanofibers were fabricated in the front side of the PT substrate(labeled as G/PT); for another sample, the nanofibers are fabricated onthe back side of the PT (labeled as PT/G). The third one is that thegelatin nanofiber layer are on both sides of the PT layer (labeled asG/PT/G). The testing side (air flow goes from) for each sample isindicated under Table 2. To compare the performance, all these sampleswere prepared with the same nanofiber areal density (ca. 2 g/m²). Thenthe results were also compared with that of the PT and neat gelatinnanofibers (2 g/m²).

In FIG. 28a , the removal efficiency for solid particles with differentsizes is compared among all the samples. For particles with sizes largerthan ca. 0.5 μm, the removal efficiency does not change significantlyamong the hybrid samples and neat gelatin, and lies in the range of98.80-99.98%, but much higher than PT sample (ca. 93%). However, forparticles with sizes smaller than 0.5 μm, in particular PM_(0.3), shownin FIG. 28b , the hybrid samples possess much higher removal efficiency(ca. 82.0%) compared with that of the neat gelatin (76.0%). In addition,PT shows much lower removal efficiencies (ca. 9%) especially for theparticles with sizes less than 5 μm. These results can be explained inthat, first, removal of smaller particles is more efficiently governedby the interaction-based mechanism between the pollutants and thenanofibers; secondly, nanofibers provide much higher surface area andsurface energy which can increase the probability of pollutant-fiberencountering; thirdly, the circulatory air flow between nanofibers andpaper towel substrate, which mentioned above, can drastically enhancethe possibility of the pollutants to encounter with the nanofibers, asshown in the SEM images of FIGS. 27a-27h . Furthermore, the G/PT/Gsample showed the highest removal efficiency of 99.30% for PM_(0.3)particles and more than 99.98% for larger particles. Therefore,enhancement in the filtration efficiency can be realized by sandwichingthe micron fibers of paper towel between two layers of nanofibers toform a hybrid layered filters. The reason is believed to be that thesandwich structure creates more circulatory air flow between nanofibersand micron fibers. The effects of nanofiber configuration on the removalefficiency for PM_(2.5) are compared in FIG. 28c . It is found that theremoval efficiency of PM_(2.5) was dramatically increased from 9.12%(for PT) to ca. 96.0% (for G/PT) by introducing the gelatin nanofiberson one side of the PT substrate. Furthermore, the G/PT/G samplepossesses the highest PM_(2.5) removal efficiency of 99.91% comparedwith all the other samples including the neat gelatin filter (ca.93.10%). These results indicate that, introducing protein nanofibers onthe PT substrate can significantly improve the efficiency for filtrationof small particulate pollutants for paper towel, fibrous structure ofpaper towel, as a substrate, can enhance the performance of thenanofilter mats for filtration of big particles.

In addition, the effect of air flow rate on the particulate airfiltration properties of the three-layer hybrid sample G/PT/G that hasthe highest removal efficiencies was studied. The results shown in FIG.28d indicate that for particles with sizes larger than 2.5 μm, theremoval efficiency has no visible change with the air flow rate. Forsmaller particles with size less than 2.5 μm, the removal efficiency isslightly affected by the air flow rate. In specific, the PM_(0.3)removal efficiency was slightly decreased from 99.30% to 98.26% with theincreased flow rate. Therefore, the removal efficiency of the hybridfilter sample was very stable for different air flow situation, which isvery positive for use as high efficiency air filters.

Removal Capability for Various Toxic Chemicals.

In addition to a high particulate filtration performance, filtrationefficiency for multiple types of toxic chemicals was also studied. Fourtypes of toxic gases with different molecular structure, such asformaldehyde (HCHO), carbon monoxide (CO), sulfur dioxide (SO₂), andvolatile organic compounds (VOCs), were chosen to test the toxicchemical removal efficiency for the samples. These chemicals arecarcinogens and very toxic gases that are present at the hazardous levelin cigarette smoke. Table 3 summarizes the toxic chemical removalefficiency of PT, neat gelatin nanofibers, and the hybrid filter samplesfor these gases. For the HCHO removal performance, the overall range isbetween 13.0% and 82.58%. In specific, compared with PT sample that hasa much lower removal efficiency (13.32%), the gelatin nanofiber-coatedPT hybrid samples present much higher filtration efficiency (ca. 77.0%for both G/T and PT/G, and 83.70% for G/PT/G). In addition, the HCHOremoval efficiency of all the three hybrid samples is higher than thatof the neat gelatin nanofiber filters (ca. 65%). These results indicatethat the combination of the gelatin nanofibers and PT substrate candramatically increase the HCHO removal capability compared with each oftheir individual component materials separately. Similarly for the COfiltration performance, the removal efficiency of all the hybrid sampleslies in a range of 69.0% to 81.0%, while the PT sample shows a removalefficiency of 20.7%, which is lower than that of pure gelatin filter(62.3%) and much lower than that of the hybrid samples. It is noted thatthe hybrid samples possess slightly lower CO removal efficiency ascompared with that of HCHO. This phenomenon can be explained in that,first, HCHO has an active aldehyde group that may interact with thefunctional groups existing in the structure of gelatin via strongchemical bonding and result in high HCHO removal efficiency; second, COis a polar molecule which can interact with the gelatin molecules via asecondary interaction type (e.g. polar-polar interaction); besides,being a small molecule enables it to penetrate through the filter mateasier.

Similarly, the hybrid samples show excellent SO₂ removal efficiency(77.85 to 81.77%), which is much higher than that of the PT (ca. 11.40%)and neat gelatin nanofibers (ca. 63.38%). In particular, the three-layerhybrid sample that has nanofibers on both sides on the PT layer showsthe highest SO₂ removal efficiency (81.77%). Regarding the VOCs, the PTsample possesses much lower filtration efficiency than that of the neatgelatin and all the hybrid filters (79.14% to 83.70%). As a comparison,the chemical removal efficiency of commercial HEPA filters for HCHO andCO is less than 5% and 3, respectively, which means the commercialfilters are incapable of removing toxic chemicals from the air, despiteof possessing much higher material areal density (ca. 164 g/m²) than ourprotein-based nanofiber mats coated on paper towel (with areal densityca. 2 g//m²). We believe that the toxic chemical removal is governed byinteraction-based mechanisms. These mechanisms are contributed by thefunctional groups existing in the molecular structures of gelatin andcellulose, since the gaseous pollutants are very small molecules andcannot be removed via the primary physical mechanisms that are veryeffective for filtration of the particulate pollutants.

Additionally, the effect of air flow rate on the toxic chemical removalefficiency of the sandwich hybrid sample (G/PT/G) was also investigated.From the results of FIG. 29, it can be observed that the chemicalremoval efficiency vs. the testing flow rate range from 4 to 10 lit/minfor all the toxic gases were very stable. These results, in addition tothe particulate filtration performance, indicate that the hybridfiltering materials can be promising for development of the highperformance “green” multifunctional air filters to be utilized invarious air filtration applications.

TABLE 3 Toxic chemical removal efficiency of air filter samples forvarious toxic chemicals Sample E_(HCHO) (%) E_(CO) (%) E_(SO2) (%)E_(VOCs) (%) PT 13.30 ± 3.24 20.70 ± 2.48 11.40 ± 2.20 27.55 ± 1.50Gelatin 65.00 ± 1.24 62.34 ± 1.38 63.38 ± 1.12 79.14 ± 1.20 Gelatin/PT77.37 ± 0.98 69.32 ± 1.10 79.85 ± 0.80 82.82 ± 1.30 PT/Gelatin 76.92 ±1.40 69.26 ± 0.75 77.94 ± 0.52 80.68 ± 0.33 Gelatin/PT/ 83.70 ± 1.1080.22 ± 1.40 81.77 ± 0.46 83.70 ± 0.55 Gelatin

Air Flow Resistance/Air Pressure Drop.

In addition to the particulate and toxic chemical capturingcapabilities, air flow resistance, or pressure drop, is anotherimportant factor for air filters regarding the air filtrationperformance. The high filtration efficiency filters, in particular HEPAgrade filters, must possess a pressure drop of less than ca. 325 Pa at astandard air face velocity. FIG. 30a shows the air flow resistancevalues for the hybrid samples in comparison with that of the PT and neatgelatin nanofiber mats. It is found that the hybrid samples possessslightly higher resistance to flow than the PT and neat gelatinnanofiber mats. This result is reasonable since by combining the gelatinnanofibers and PT fibers, air must penetrate through more layers offibers, which leads to increased pressure drop to some extent. Moreover,it is found that the configuration of the nanofibers plays an importantrole in changing the pressure drop of the filter. As it can be seen thatthe sample with nanofibers in the front of PT substrate (G/PT) have apressure drop of ca. 147 Pa compared with that of the sample withopposite nanofiber configuration (ca. 142 Pa). This is due to that thepore structure of the mat affects the pressure drop of the filter. Inthis study, the nanofiber mats have smaller pore size (ca. 4.2 μm forgelatin nanofibers) than that of the micrometer size fiber mats (ca. 58μm for PT). Thereby, when the gelatin nanofibers is coated in the frontside of the PT, the air stream has to penetrate through the smallerpores of nanofibers first, while for the opposite nanofiberconfiguration, air stream can easily breach through the large pores ofPT. This flow sequence can result in a change in the air flow pathwaysthrough the entire filter which can lead to the change in pressure drop.

Furthermore, the effect of air flow rate on the pressure drop for theG/PT/G filter mat was tested and the results are shown in FIG. 30b . Itis found that the pressure drop normally increases from 168 Pa to 195 Paas the air flow rate rises from 4 lit/min to 10 lit/min. This result isowing to the multi-layers of pore and fiber structures in the sandwichhybrid filter, which can cause the change in the air stream pathwayswhen it passes through the multiple layers of nanometer and micrometersize fibers. Although the pressure drop of the hybrid filter slightlyincreased with the air flow rate, the value (200 Pa) is still well belowthe DoE standard requirement of the high efficiency air filters (325Pa). By taking the filtration performance, such as for particulatematter (FIGS. 28a-28d ), toxic chemical removal efficiencies (FIG. 29),and the pressure drop values (FIGS. 30a and 30b ) into account, it canbe concluded that, coating paper towel substrate with (a) thin layer(s)of protein nanofibers can provide high efficiency and multifunctional“green” air filtering materials which have great potential to beutilized in various air filtration applications.

3.3. Hybrid Filters: Soy Protein Composite Nanofiber-Coated Paper Towel

Our previous work demonstrated that the soy protein compositenanofabrics possess extremely high removal efficiencies for bothparticulate and toxic chemical pollutants. We reported a PM_(2.5)removal efficiency is more than 99.50% and toxic chemical capturingefficiency is more than 70% for the soy composite nanofiber filters withSPI to PVA ratio as 1:1 and has 4.50 g/m² areal density. Therefore, athin layer of such composite nanofibers prepared under the sameelectrospinning conditions were coated onto the paper towel substrate(procedures as shown FIG. 26) and the air filtration properties of thisnovel hybrid filter were investigated.

Morphology of the SC/PT Filter.

First, the morphology of the SC/PT hybrid filter mat with nanofiberareal density of ca. 2 g/m² was studied using SEM technique. FIG. 31ashows the SEM image of SC/PT fibers before filtration testing. It can beobserved that the soy protein composite nanofibers are uniformlydeposited on the PT substrate. These nanofibers possess an average fiberdiameter of ca. 143 nm and very uniform fiber diameter and pore sizedistribution. FIG. 31b shows the SC/PT filter after the filtrationtesting using cigarette smoke as the polluted air. It can be seen thatthe nanofibers captured large amount of pollutants from the air, as thenanofibers possess very high surface area and surface energy, whichenables the SC/PT hybrid filter to remove more pollutants. Additionally,the hybrid SC/PT filter sample also showed obvious color change fromwhite (before filtration) to a yellowish color (after filtration), whichis similar to the change of the hybrid gelatin/PT filter samples. Thiscolor change, along with the air filtration results shown in FIGS. 32aand 32b , indicates that the nanofibers captured various pollutantsincluding both solid particles and gaseous chemicals from the air.

Particulate and Chemical Filtration Performance.

In order to find out the effect of SC nanofiber layer on the filtrationproperties, the particulate and chemical removal efficiency of thehybrid filter were studied. The removal efficiency of PT, pure SCnanofibers, and SC/PT hybrid filter for solid particulate pollutantswith different sizes are compared in FIG. 32a . The removal efficiencyfor the large particles (larger than 0.5 μm) of the hybrid SC/PT sampleis slightly higher than that of pure SC nanofibers, and lies in therange of 99.60-99.82%, but it is much higher than that of the PT sample(ca. 93%). For smaller particles, this hybrid sample presented higherremoval efficiency (ca. 88.90%) compared with that of the soy proteincomposite nanofibers (84.58%) and PT (ca. 9%), in particular forPM_(0.3). It is believed that these results is because of the creationof circulatory air flow between the nanofibers and paper towelsubstrate, which can significantly increase the time and chance for thenanofibers to encounter with the pollutants and remove them from theair.

Furthermore, the removal capability of the SC/PT hybrid filter for fourdifferent toxic gaseous chemicals including HCHO, CO, SO₂, and VOCs, wasstudied. In FIG. 32b , the removal efficiency of the SC/PT hybrid filterfor these chemicals is compared with that of the PT and pure SCnanofiber mat. For the HCHO removal capability, the three samples havean efficiency range from 13.0% to 56%. The SC/PT hybrid sample possessesthe highest filtration efficiency (ca. 55.32%) compared with that of thePT (ca. 13.32%) and pure SC nanofabrics (ca. 46.12%). In addition, forCO removal capability, the SC/PT hybrid sample presents a removalefficiency as high as 85.33%, which is higher than that of the pure SCnanofabrics (ca. 78.34%) and much higher than that of the PT (ca.20.7%). Similarly, the SC/PT hybrid sample shows the highest SO₂ (ca.62.13%) and VOC (ca. 79.75%) removal efficiency among all three samples.These values are much higher than that of the pure SC nanofabrics (ca.55.38% and ca. 77.14% for SO₂ and VOC, respectively) and the PT (ca.11.4% and 27.55% for SO₂ and VOC, respectively). These results, alongwith the particulate removal performance of the SC/PT hybrid filter mat,indicate that the combination of soy protein-based nanofibers and papertowel substrate can lead to excellent filtration performance of for thehybrid filter compared with the filters made of each componentindividually.

3.4. Toxic Chemical Filtration Mechanisms

In order to understand the outstanding filtration performance of theprotein nanofiber-coated PT materials, an interaction-based filtrationmechanism, in addition to the primary physical capturing mechanisms, isproposed. This mechanism is projected based on the chemicalcharacteristics of the filtering materials (proteins and cellulose) aswell as that of pollutants. Typically, the conventional filter fiberscapture the particulate pollutants only via the primary physicalfiltration mechanisms (size effects), which are governed by the fibermat porous morphology. Therefore, the combination of protein-basednanofibers and micron cellulose fibers leads to a special porousstructure with extremely higher surface area/energy and huge amount offunctional groups that can dramatically enhance the fiber-pollutantinteractions. Furthermore, the paper towel substrate can change the airflow pathways and create a circulatory flow between the protein-basednanofibers and cellulose micron-fiber in paper towel as shown in FIG.33a . This phenomenon can increase the possibility and chances for thepollutants to encounter with the nanofibers and, as a result, tosignificantly increase in the removal efficiency. Moreover, thebiomaterials, e.g. cellulose and natural protein, include variousfunctional groups such as methylol, hydroxyl, glycine, proline,hydroxyproline, glutamic acid, lysine, and etc. in their structures.These functional groups offer many sites for strong interactions betweenthe fibers and various pollutants, which enable the hybrid samples,protein nanofiber-coated PT filters, to achieve very high filtrationefficiencies for both particulate and toxic gaseous chemicals. FIG. 33bshows the schematic interactions between the filtering materials andvarious pollutants such as PM particles, SO₂, HCHO and CO. The PMparticles with different surface chemistries can undergo various typesof interactions such as hydrogen bonding, ionic interactions,hydrophobic-hydrophobic interactions and etc. These interactions aremore effective than the size effect mechanisms via the porous structuresand can dramatically enhance the filtration performance of the filter.More significantly, gaseous pollutants, such as SO₂ and CO, can undergodipole-dipole interactions with the protein nanofibers due to the unevendistribution of charges (electronegativity difference) on theirmolecules. Also, functional sites in the structures of chemicals, suchas HCHO, can interact with the amine and carboxyl groups of the proteinand/or cellulose which may lead to chemical absorptions and formation ofnew bonds (e.g. aldamine linkage). This phenomenon may be one of thereasons behind the color change of the filter from white to yellowishshown in FIG. 27h ) after the filtration testing. This schematic figureillustrates that the fiber-pollutant interaction is important inunderstanding the filtration mechanism of the multifunctional airfilters.

To prove the new interaction-based filtration mechanism, Fouriertransform infrared spectroscopy (FTIR) was employed to examine thechange of pure SO₂ gas before and after filtration. FIG. 33c shows aschematic illustration of the testing apparatus for pure SO₂ filtrationand FTIR characterization. Firstly, the FTIR gas chamber was vacuumed,and then filled with SO₂ gas and the FTIR spectra of pure SO₂ wascollected; secondly, the same amount of SO₂ passed through the filter,and then was collected in the FTIR gas chamber. FIG. 33d shows the FTIRspectra of the pure SO₂ before and after the filtration via using thepaper towel as the filter media. The main peaks for the pure SO₂ arearound 521, 1158, 1360, and 2514 cm⁻¹ which correspond to S—O bending,symmetric stretching, asymmetric stretching, and linear combinations ofchanges in the bonding length and angels, respectively. It can be seenthat the intensity of these peaks does not change significantly(decreased less than 10%) after passing through the paper towel filtermat. This result is reasonable because the cellulose fiber of the papertowel only possesses active hydroxyl and carboxyl groups that caninteract with the SO₂ molecules. By comparison of the FTIR spectra ofthe SO₂ before and after passing through the G/PT filter (shown in FIG.33e ), one can easily identify the dramatic changes in the SO₂ mainpeaks. It is found that the S—O bond bending, symmetric stretching, andlinear combination peaks completely disappeared. Also, the intensity ofthe asymmetric S—O stretching was decreased by more than 80%. Theseresults can be explained in that the gelatin protein possesses numerousactive functional groups, such as amine, carboxyl, hydroxyl, and chargedgroups, which can strongly interact and immobilize the SO₂ molecules viadipole-dipole interactions and hydrogen bonding. These FTIR results areconsistent with the toxic chemical removal efficiency values discussedabove. Therefore, the study demonstrates the presence of stronginteractions between gelatin nanofibers and various pollutants,particularly toxic gaseous chemicals, in the polluted air. Theseinteractions can lead to the interaction-based filtration mechanisms,alongside with the primary physical capturing mechanism (size effects),which are effective for capturing different particles and toxicchemicals and, as a result, enhancing the multifunctional filtrationperformance.

Conclusions

The hybrid protein nanofiber-cellulose micro-fiber structures, i.e.protein nanofiber-coated PT filters (both gelatin-based and soy proteincomposite-based with paper towel as the substrate) presented very highfiltration efficiencies for both particulate pollutants with variety ofsizes and multiple toxic chemicals. Paper towel engaged as a substratefor protein nanofibers further enhance filtration performance, which canlead to the development of an environmentally friendly, sustainable andlow cost air filtering materials. More significantly, this work alsodemonstrates that the functional groups in the structure of bothcellulose and proteins can provide active sites to interact with varioustypes of pollutants. It is found that the functional groups in thenatural polymer structures are crucial for capturing the toxic chemicalsand particles, in particular, for the particles with sizes smaller thanthe pore size of the filter mats. Additionally, the hybrid filters madeof protein nanofiber-cellulose paper towel presented an extremely highmultifunctional filtration performance while possessing a low pressuredrop, both of which are most important parameters for feasibleapplications. In conclusion, this work indicates a class of highefficiency and multifunctional “green” air filtering materials generatedfrom natural protein nanofiber-coated cellulose paper towel. Thecombination of protein nanofibers with the cellulose microfiber mat(paper towel) as the substrate provides a favorable environmentallyfriendly material system with excessive potential to capture variouseven unexpected pollutants from the air owing to their advancedinteraction-based filtration mechanism.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An air filter,comprising a porous nanofiber mat configured to filter particles havinga diameter of about 0.1 μm or greater when air is passed through the airfilter, the porous nanofiber mat comprising a plurality ofprotein-containing nanofibers, comprising a protein configured to bindto, and thereby filter, at least one chemical species.
 2. The air filterof claim 1, wherein at least a portion of the plurality ofprotein-containing nanofibers consist essentially of protein.
 3. The airfilter of claim 2, wherein the protein is selected from the groupconsisting of plant-based proteins, animal-based proteins, and syntheticproteins.
 4. The air filter of claim 1, wherein at least a portion ofthe plurality of protein-containing nanofibers are composite nanofiberscomprising protein and a polymer.
 5. The air filter of claim 4, whereinthe protein is selected from the group consisting of plant-basedproteins, animal-based proteins, and synthetic proteins.
 6. The airfilter of claim 4, wherein the polymer is selected from the groupconsisting of poly(vinyl alcohol) (PVA), poly(ethylene oxide) (PEO),poly(acrylonitrile) (PAN), and nylon.
 7. The air filter of claim 4,wherein the ratio of protein to polymer, by weight, is in the range of0.5:1 to 2:1.
 8. The air filter of claim 1, wherein the at least onechemical species is selected from the group consisting of carbonmonoxide, formaldehyde, Sulfur oxides (SO_(x)), nitrogen oxides(NO_(x)), Ammonia (NH₃), carbon dioxide (CO₂), volatile organicchemicals (VOCs), Ozone (O₃).
 9. The air filter of claim 1, wherein theair filter is configured for air flow through the air filter such thatthe resistance to air flow is 250 Pa or less at 4 L/min air flow rate.10. The air filter of claim 1, wherein the porous nanofiber mat has athickness in the range of 8 μm to 30 μm.
 11. The air filter of claim 1,wherein the plurality of protein-containing nanofibers have an averagediameter of 1000 nm or smaller.
 12. The air filter of claim 1, whereinthe plurality of protein-containing nanofibers consist essentially ofprotein and have an average diameter of 100 nm or smaller.
 13. The airfilter of claim 1, wherein the plurality of protein-containingnanofibers are composite nanofibers comprising protein and a polymer andhave an average diameter of 150 nm or smaller.
 14. The air filter ofclaim 1, wherein the plurality of protein-containing nanofibers have aGaussian diameter size distribution.
 15. The air filter of claim 1,further comprising a cellulose-fiber layer that is adjacent to orabutting the porous nanofiber mat.
 16. The air filter of claim 15,wherein the cellulose fiber layer provides a mechanical support for theporous nanofiber mat and is configured to filter particles from the airpassed through the air filter.
 17. The air filter of claim 15, furthercomprising a second porous nanofiber mat.
 18. The air filter of claim17, wherein the second porous nanofiber mat is adjacent or abutting thecellulose-fiber layer on an opposite side in relation to the porousnanofiber mat.
 19. The air filter of claim 17, wherein the compositionand configuration of the second porous nanofiber mat are the same as theporous nanofiber mat.
 20. The air filter of claim 17, wherein thecomposition or configuration of the second porous nanofiber mat isdifferent than the porous nanofiber mat.